Imaging device, imaging module, electronic device, and imaging system

ABSTRACT

An imaging device connected to a neural network is provided. An imaging device having a neuron in a neural network includes a plurality of first pixels, a first circuit, a second circuit, and a third circuit. Each of the plurality of first pixels includes a photoelectric conversion element. The plurality of first pixels is electrically connected to the first circuit. The first circuit is electrically connected to the second circuit. The second circuit is electrically connected to the third circuit. Each of the plurality of first pixels generates an input signal of the neuron. The first circuit, the second circuit, and the third circuit function as the neuron. The third circuit includes an interface connected to the neural network.

TECHNICAL FIELD

One embodiment of the present invention relates to an imaging device, animaging module, an electronic device, and an imaging system.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. The present inventionrelates to a process, a machine, manufacture, or a composition ofmatter. In particular, one embodiment of the present invention relatesto a semiconductor device, a display device, a light-emitting device, apower storage device, a memory device, a driving method thereof, or amanufacturing method thereof.

In this specification and the like, a semiconductor device refers to anelement, a circuit, a device, or the like that can function by utilizingsemiconductor characteristics. An example of the semiconductor device isa semiconductor element such as a transistor or a diode. Another exampleof the semiconductor device is a circuit including a semiconductorelement. Another example of the semiconductor device is a deviceprovided with a circuit including a semiconductor element.

BACKGROUND ART

As a semiconductor material applicable to a transistor, an oxidesemiconductor has been attracting attention. For example, a techniquefor forming transistors using zinc oxide or an In—Ga—Zn-based oxidesemiconductor as an oxide semiconductor is disclosed in Patent Document1.

Patent Document 2 discloses an imaging device in which a transistorincluding an oxide semiconductor is used in part of a pixel circuit.

Patent Document 3 discloses an imaging device in which a transistorincluding silicon, a transistor including an oxide semiconductor, and aphotodiode including a crystalline silicon layer are stacked.

A neural network, which has learning ability, is excellent innonlinearity and pattern matching performance and applied to a varietyof fields of, for example, control, a forecast, and diagnosis. Manytypes of structure of the neural network have been proposed. Most of theneural networks that have been put into practical use have a three-layerhierarchical architecture in which two layers (intermediate layer andoutput layer) of neuron elements each having the sigmoid function arestacked. This is because there is evidence that the three-layerhierarchical architecture can model any function with arbitraryaccuracy.

Patent Document 4 proposes an information system that extracts anddetects an object from an image obtained with an imaging device.

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2011-119711-   [Patent Document 3] Japanese Published Patent Application No.    2013-243355-   [Patent Document 4] Japanese Published Patent Application No.    2014-032542

DISCLOSURE OF INVENTION

A semiconductor integrated circuit with high density and large capacityhas been developed; meanwhile, miniaturization of the semiconductorintegrated circuit is required. Thus, two-dimensional integration hasshifted to three-dimensional integration. Although a manufacturingprocess of three-dimensional integration structure might be complicated,the degree of freedom of the materials and the design rule of layers canbe increased. In view of the above, it is an object to manufacture ahighly functional semiconductor integrated circuit that is difficult tomanufacture by two-dimensional integration.

A pixel of an imaging device includes a photoelectric conversion elementand a transistor. The photoelectric conversion element needs to havehigh optical sensitivity, and the transistor needs to have a smalloff-state current and low noise characteristics. It is an object tomanufacture a more highly functional imaging element in which aphotoelectric conversion element and a transistor are integratedthree-dimensionally and which is manufactured using materials suitablefor the photoelectric conversion element and the transistor inmanufacturing steps.

Peripheral circuits such as driver circuits are preferably manufacturedin the same process as the pixels so that a connection step and the likeare simplified.

Recognition and determination by artificial intelligence (AI) of animage taken with an imaging device has been studied. Artificialintelligence aims to realize several characteristics of human brainfunctions by a neural network, and requires a large amount of arithmeticoperations. In view of the above, it is an object to efficiently performan arithmetic operation with a neural network in hardware.

An object of one embodiment of the present invention is to compress andconvert a plurality of pieces of pixel data into one piece of dataincluding one feature. Another object is to increase the arithmeticoperation speed of an imaging device. Another object is to provide animaging device that is integrated three-dimensionally. Another object isto provide an imaging device in which deterioration of a signalgenerated through conversion by a photoelectric conversion element canbe reduced. Another object is to provide a novel imaging device or thelike.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

Note that the objects of one embodiment of the present invention are notlimited to the above objects. The objects described above do not disturbthe existence of other objects. The other objects are the ones that arenot described above and will be described below. The other objects willbe apparent from and can be derived from the description of thespecification, the drawings, and the like by those skilled in the art.One embodiment of the present invention is to achieve at least one ofthe aforementioned objects and the other objects.

One embodiment of the present invention is an imaging device, which hasa neuron in a neural network, including a plurality of first pixels, afirst circuit, a second circuit, and a third circuit. Each of theplurality of first pixels includes a photoelectric conversion element.The photoelectric conversion element is electrically connected to thefirst circuit. The first circuit is electrically connected to the secondcircuit. The second circuit is electrically connected to the thirdcircuit. Each of the plurality of first pixels generates an input signalof the neuron in the neural network. The first circuit, the secondcircuit, and the third circuit have functions as the neuron. The thirdcircuit includes an interface connected to the neural network.

In the above structure, each of the plurality of first pixels ispreferably configured to convert received light into an analog signal.The first circuit is preferably configured to amplify the analog signal.The second circuit is preferably configured to add the analog signalsthat are amplified. The third circuit is preferably configured toconvert the analog signals that are added into feature data by using anactivation function. The third circuit is preferably configured todetermine the feature data.

In the above structure, the first circuit preferably includes anamplifier circuit, a first memory circuit, and a first adder circuit.The second circuit preferably includes a second adder circuit. The thirdcircuit preferably includes a first arithmetic circuit and a secondmemory circuit. The first pixel is preferably configured to convertlight into a first signal to output. The amplifier circuit is preferablyconfigured to amplify the first signal at an amplification factor heldin the first memory circuit. The first adder circuit is preferablyconfigured to add an offset voltage to the first signal that isamplified. The first adder circuit is preferably configured to outputthe result of addition of the offset voltage as a second signal. Thesecond adder circuit is preferably configured to add the second signals.The second adder circuit is preferably configured to output a signalthat is obtained by adding the second signals as a third signal that isan analog signal. The first arithmetic circuit is preferably configuredto determine and binarize the third signal. The first arithmetic circuitis preferably configured to supply the signal that is binarized to thesecond memory circuit as the feature data. The second memory circuitpreferably outputs the feature data to the neural network.

In the above structure, each of the plurality of first pixels ispreferably configured to convert received light into an analog signaland output the analog signal as a fourth signal. The first circuit ispreferably configured to convert the analog signal into a digitalsignal. The first circuit is configured to generate a fifth signalhaving a feature by classifying a level of the digital signal using bitshift. The second circuit is preferably configured to extract and countthe feature of the fifth signal. The third circuit is preferablyconfigured to convert a result of count into the feature data by usingan activation function. The third circuit is preferably configured todetermine the feature data.

In the above structure, the first circuit preferably includes a firstinput selection circuit, an analog-to-digital converter circuit, a firstdetermination circuit, and a first memory circuit. The second circuitpreferably includes a second input selection circuit and a featureextraction circuit. The third circuit preferably includes a seconddetermination circuit and a second memory circuit. The first inputselection circuit is preferably configured to select any of a pluralityof fourth signals. The analog-to-digital converter circuit is preferablyconfigured to convert the fourth signal that is a selected analog signalinto a digital signal. The first determination circuit is preferablyconfigured to amplify the digital signal by a power of two in accordancewith an amount of bit shift that is selected. The first determinationcircuit is preferably configured to determine a level of the signal thatis amplified, in accordance with the amount of bit shift. The firstdetermination circuit is preferably configured to supply thedetermination result to the first memory circuit as the fifth signal.The second input selection circuit is preferably configured tosequentially select the fifth signal held in the first memory circuitand output the selected holding data to the feature extraction circuit.The feature extraction circuit is preferably configured to count thefifth signal with a feature. The second determination circuit preferablycompares the count result with a supplied condition. The seconddetermination circuit is preferably configured to supply the comparisonresult to the second memory circuit as feature data. The second memorycircuit preferably outputs the feature data to the neural network.

In the above structure, it is preferable that the imaging device havingthe neuron in the neural network further comprise a signal line and asecond analog-to-digital converter circuit. Each of the plurality offirst pixels is preferably configured to convert received light into ananalog signal. The analog signal is preferably supplied to the secondanalog-to-digital converter circuit through the signal line from each ofthe plurality of first pixels.

In the above structure, the third circuit preferably includes aselection circuit. The feature data is preferably divided into pieces ofdata each having a selected length and output to the neural network.

In the above structure, the first pixel preferably includes a firsttransistor. The first transistor preferably includes a metal oxide in asemiconductor layer.

In the above structure, the first transistor included in the first pixelpreferably includes the metal oxide in the semiconductor layer. A secondtransistor included in a circuit preferably comprises polycrystallinesilicon in a semiconductor layer.

In the above structure, the first transistor including the metal oxidein the semiconductor layer preferably includes a back gate.

In the above structure, the first transistor preferably includes aregion overlapping with the photoelectric conversion element.

According to one embodiment of the present invention, a plurality ofpieces of pixel data can be compressed and converted into one piece ofdata including one feature. The arithmetic operation speed of an imagingdevice can be increased. An imaging device that is integratedthree-dimensionally can be provided. An imaging device in whichdeterioration of a signal generated through conversion by aphotoelectric conversion element can be reduced can be provided. A novelimaging device or the like can be provided.

Note that the effects of one embodiment of the present invention are notlimited to the above effects. The effects described above do not disturbthe existence of other effects. The other effects are the ones that arenot described above and will be described below. The other effects willbe apparent from and can be derived from the description of thespecification, the drawings, and the like by those skilled in the art.One embodiment of the present invention is to have at least one of theaforementioned effects and the other effects. Therefore, one embodimentof the present invention does not have the effects described above insome cases.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an imaging element.

FIGS. 2A and 2B are block diagrams illustrating an imaging element.

FIGS. 3A to 3C are circuit diagrams illustrating an imaging element.

FIG. 4A is a timing chart showing an operation of an imaging element,and FIG. 4B is a timing chart showing an operation of a pixel.

FIG. 5 is a block diagram illustrating an imaging element.

FIGS. 6A and 6B are block diagrams illustrating an imaging element.

FIG. 7 is a circuit diagram illustrating an imaging element.

FIG. 8A is a timing chart showing an operation of an imaging element,and FIG. 8B is a timing chart showing an operation of a pixel.

FIG. 9 is a cross-sectional view illustrating a structure of an imagingdevice.

FIG. 10 is a cross-sectional view illustrating a structure of an imagingdevice.

FIGS. 11A to 11E are cross-sectional views each illustrating connectionof a photoelectric conversion element.

FIGS. 12A to 12D are cross-sectional views each illustrating connectionof a photoelectric conversion element.

FIG. 13 is a cross-sectional view illustrating a structure of an imagingdevice.

FIGS. 14A to 14C are cross-sectional views each illustrating connectionof a photoelectric conversion element.

FIG. 15 is a cross-sectional view illustrating a structure of an imagingdevice.

FIGS. 16A and 16B are cross-sectional views each illustrating astructure of an imaging device.

FIG. 17 is a cross-sectional view illustrating a structure of an imagingdevice.

FIGS. 18A to 18C are circuit diagrams each illustrating a pixel.

FIGS. 19A and 19B are circuit diagrams each illustrating a pixel.

FIG. 20A is a block diagram of an analog-to-digital converter circuit,and FIG. 20B illustrates connection between an imaging element and theanalog-to-digital converter circuit.

FIG. 21A is a top view of a transistor, and FIGS. 21B and 21C arecross-sectional views thereof.

FIG. 22A is a top view of a transistor, and FIGS. 22B and 22C arecross-sectional views thereof.

FIG. 23A is a top view of a transistor, and FIGS. 23B and 23C arecross-sectional views thereof.

FIG. 24A is a top view of a transistor, and FIGS. 24B and 24C arecross-sectional views thereof.

FIG. 25A is a top view of a transistor, and FIGS. 25B and 25C arecross-sectional views thereof.

FIG. 26A is a top view of a transistor, and FIGS. 26B and 26C arecross-sectional views thereof.

FIG. 27A is a top view of a transistor, and FIGS. 27B and 27C arecross-sectional views thereof.

FIGS. 28A to 28H are cross-sectional views and top views of transistors.

FIGS. 29A to 29D are perspective views and a cross-sectional viewillustrating a package including an imaging device.

FIGS. 30A to 30D are perspective views and a cross-sectional viewillustrating a package including an imaging device.

FIGS. 31A to 31F illustrate electronic devices.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented with various modes. It willbe readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, the size, the layerthickness, or the region is not limited to the illustrated scale. Notethat the drawings are schematic views showing ideal examples, andembodiments of the present invention are not limited to shapes or valuesshown in the drawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

In this specification, terms for describing arrangement, such as “over”,“above”, “under”, and “below”, are used for convenience in describing apositional relation between components with reference to drawings.Furthermore, the positional relation between components is changed asappropriate in accordance with a direction in which each component isdescribed. Thus, there is no limitation on terms used in thisspecification, and description can be made appropriately depending onthe situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. The transistorhas a channel formation region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow between thesource and the drain through the channel formation region. Note that inthis specification and the like, a channel formation region refers to aregion through which current mainly flows.

Furthermore, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through an“object having any electric function”. There is no particular limitationon the “object having any electric function” as long as electric signalscan be transmitted and received between components that are connectedthrough the object. Examples of an “object having any electric function”are a switching element such as a transistor, a resistor, an inductor, acapacitor, and an element with a variety of functions as well as anelectrode and a wiring.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. The term “perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 80° and less thanor equal to 100°, and accordingly also includes the case where the angleis greater than or equal to 85° and less than or equal to 95°.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

Unless otherwise specified, off-state current in this specification andthe like refers to drain current of a transistor in an off state (alsoreferred to as a non-conducting state and a cutoff state). Unlessotherwise specified, the off state of an n-channel transistor means thatthe voltage between its gate and source (V_(gs): gate-source voltage) islower than the threshold voltage V_(th), and the off state of ap-channel transistor means that the gate-source voltage V_(gs) is higherthan the threshold voltage V_(th). For example, the off-state current ofan n-channel transistor sometimes refers to drain current that flowswhen the gate-source voltage V_(gs) is lower than the threshold voltageV_(th).

The off-state current of a transistor depends on V_(gs) in some cases.Therefore, “the off-state current of a transistor is I or smaller” maymean that the off-state current of the transistor is I or smaller at acertain V_(gs). The off-state current of a transistor may refer tooff-state current at a given V_(gs), at V_(gs) in a given range, atV_(gs) at which sufficiently small off-state current is obtained, or thelike.

As an example, an assumption is made that an n-channel transistor has athreshold voltage V_(th) of 0.5 V and a drain current of 1×10⁻⁹ A atV_(gs) of 0.5 V, 1×10⁻¹³ A at V_(gs) of 0.1 V, 1×10⁻¹⁹ A at V_(gs) of−0.5 V, and 1×10⁻²² A at V_(gs) of −0.8 V. The drain current of thetransistor is 1×10⁻¹⁹ A or smaller at V_(gs) of −0.5 V or at V_(gs) inthe range of ˜0.8 V to −0.5 V; therefore, it may be said that theoff-state current of the transistor is 1×10⁻¹⁹ A or smaller. Since thedrain current of the transistor is 1×10⁻²² A or smaller at a certainV_(gs), it may be said that the off-state current of the transistor is1×10⁻²² A or smaller.

In this specification and the like, the off-state current of atransistor with a channel width W is sometimes represented by a currentvalue per channel width W or by a current value per given channel width(e.g., 1 μm). In the latter case, the off-state current may berepresented by current per length (e.g., A/μm).

The off-state current of a transistor depends on temperature in somecases. Unless otherwise specified, the off-state current in thisspecification may be off-state current at room temperature, 60° C., 85°C., 95° C., or 125° C. Alternatively, the off-state current may beoff-state current at a temperature at which the reliability of asemiconductor device or the like including the transistor is ensured ora temperature at which the semiconductor device or the like includingthe transistor is used (e.g., a temperature in the range of 5° C. to 35°C.). The state in which the off-state current of a transistor is I orsmaller may indicate that the off-state current of the transistor atroom temperature, 60° C., 85° C., 95° C., 125° C., a temperature atwhich the reliability of a semiconductor device or the like includingthe transistor is ensured, or a temperature at which the semiconductordevice or the like including the transistor is used (e.g., a temperaturein the range of 5° C. to 35° C.) is I or smaller at a certain V_(gs).

The off-state current of a transistor depends on the voltage Vas betweenits drain and source in some cases. Unless otherwise specified, theoff-state current in this specification may be off-state current at Vasof 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V,or 20 V. Alternatively, the off-state current may be off-state currentat Vas at which the reliability of a semiconductor device or the likeincluding the transistor is ensured or at Vas used in the semiconductordevice or the like including the transistor. The state in which theoff-state current of a transistor is I or smaller may indicate that theoff-state current of the transistor at Vas of 0.1 V, 0.8 V, 1 V, 1.2 V,1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V, at Vas at which thereliability of a semiconductor device or the like including thetransistor is ensured, or at Vas used in the semiconductor device or thelike including the transistor is I or smaller at a certain V_(gs).

In the above description of the off-state current, a drain may bereplaced with a source. That is, the off-state current sometimes refersto current that flows through a source of a transistor in the off state.

In this specification and the like, the term “leakage current” sometimesexpresses the same meaning as “off-state current”. In this specificationand the like, the off-state current sometimes refers to current thatflows between a source and a drain of a transistor in the off state, forexample.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or a voltage, and “potential” and “voltage” are usedas synonymous words in many cases. Therefore, in this specification,“potential” can be replaced with “voltage” and vice versa, unlessotherwise specified.

Embodiment 1

In this embodiment, an imaging device that includes an interfaceconnected to a neural network is described with reference to FIG. 1 ,FIGS. 2A and 2B, FIGS. 3A to 3C, and FIGS. 4A and 4B.

One embodiment of the present invention is a structure and an operatingmethod of an imaging device in which determination circuits for signalsoutput from pixels are distributed in an imaging element.

FIG. 1 is a block diagram illustrating a configuration example of animaging device 100. The imaging device 100 includes an imaging element10, an analog-to-digital converter circuit (hereinafter referred to asA/D converter circuit) 26, a decoder circuit 27, a selector circuit 28,and a control portion 29.

The imaging element 10 includes a plurality of determination circuits 20a, a plurality of scan lines G1, a plurality of scan lines G2, aplurality of signal lines OUT, and a plurality of signal lines OUT1.

The determination circuit 20 a includes a plurality of pixels 20, afeature extraction circuit 30, and a determination output circuit 31.The pixel 20 includes a light-receiving circuit 21 having aphotoelectric conversion element PD (see FIG. 3A).

The imaging element 10 includes the pixels 20 arranged in m rows and ncolumns. FIG. 1 illustrates part of the imaging device 10. A structurein which the pixels 20 (Pix(i, j) to Pix(i+3, j+3)) are arranged isdescribed as an example. Note that i is a natural number of 1 or moreand m or less, j is a natural number of 1 or more and n or less, m is anatural number of 2 or more, n is a natural number of 2 or more, and kis a natural number of 1 or more and n or less.

The determination circuit 20 a includes four pixels 20, the featureextraction circuit 30, and the determination output circuit 31. Thedetermination output circuit 31 includes an arithmetic circuit 31 a anda memory circuit 31 b (see FIG. 3B).

The pixels 20 are electrically connected to the feature extractioncircuit 30. The feature extraction circuit 30 is electrically connectedto the determination output circuit 31.

Note that the number of pixels 20 included in the determination circuit20 a is preferably determined as appropriate in accordance with a regionto be determined. A light-receiving 30 circuit 21 may be connected to aplurality of amplifier circuits 22 (see FIG. 2B).

The pixels Pix(i, j) and Pix(i, j+1) are electrically connected to thesignal line OUT1(i), and the pixels Pix(i+1, j) and Pix(i+1, j+1) areelectrically connected to the signal line OUT1(i+1). The pixels Pix(i,j) and Pix(i+1, j) are electrically connected to the scan line G1(j),and the pixels Pix(i, j+1) and Pix(i+1, j+1) are electrically connectedto the scan line G1(j+1). The determination output circuit 31 iselectrically connected to the signal line OUT(i).

The pixels 20 and the determination circuit 20 a can each be formedusing transistors with the same conductivity and can be manufactured inparallel without an increase in the number of steps.

The photoelectric conversion element PD included in the pixel 20 canconvert received light into current and then convert the current intovoltage. The pixel 20 can amplify the voltage of an analog signal andoutput the resulting voltage as an output signal b.

A plurality of output signals b is subjected to arithmetic operation inthe feature extraction circuit 30. Addition or multiplication ispreferably performed as the arithmetic operation. In this embodiment,the feature extraction circuit 30 is an adder circuit. The featureextraction circuit 30 can output an output signal c that is an analogsignal.

In the determination output circuit 31, the output signal c supplied toan input terminal can be determined and binarized by the arithmeticcircuit 31 a. The binarized signal can be held in the memory circuit 31b as a digital signal (see FIG. 3B).

The memory circuit 31 b outputs an output signal d. The output signal dis supplied to the selector circuit 28 through the signal line OUT (seeFIG. 1 ). The selector circuit 28 can sort the determination results ofthe determination circuits 20 a in order of required data length andtransfer them to the control portion 29. The data length of the outputsignal d can be determined in accordance with a communication method,such as parallel communication, serial communication (e.g., I2C), ordifferential communication (e.g., MIPI).

The pixel 20 can supply an output signal a to the A/D converter circuit26 through the signal line OUT1 (see FIG. 2A). The A/D converter circuit26 can perform digital conversion on the output signal a and output theresulting signal to the control portion 29. The signal can 30 betransmitted from the A/D converter circuit 26 to the control portion 29by an optimal method.

The control portion 29 includes two input interfaces. One of the inputinterfaces includes a digital interface and covers parallel input orserial input. The data length of input data is fixed. The output signala is supplied to the digital interface of the control portion 29 throughthe A/D converter circuit 26.

The other input interface covers input of a neural network. Input datais directly input to the neural network, so that the data length of theinput data is preferably changed to a data length which can be easilyhandled in the neural network. The data length of the output signal dcan be changed by the selector circuit 28. The output signal d isprocessed to have an appropriate data length and then supplied to theneural network interface of the control portion 29.

The type of photoelectric conversion element in the imaging device 100can be freely selected. For example, over a single crystal siliconsubstrate provided with a photodiode, the pixels 20 and thedetermination circuits 20 a can be formed using transistors eachincluding an oxide semiconductor in a semiconductor layer.

The transistor including an oxide semiconductor in a semiconductor layerhas a small off-state current and therefore facilitates construction ofa floating node, a latch, and a memory for retaining data in the pixel20 and the determination circuit 20 a. Thus, the semiconductor layer ofthe transistor can be selected depending on the required function.

Although the imaging device can be formed only using transistors havingthe same conductivity, the area of the imaging device might be large.For this reason, transistors each including an oxide semiconductor in asemiconductor layer are preferably used for a pixel or a memory circuit.Transistors each including single crystal silicon in a semiconductorlayer can be used for circuits that require current supply capability,such as the amplifier circuit, the determination circuit 20 a, the A/Dconverter circuit 26, and the decoder circuit 27. A transistor includingan oxide semiconductor in a semiconductor layer may be stacked over atransistor including single crystal silicon in a semiconductor layer.Note that an example of the oxide semiconductor will be described inEmbodiment 6 in detail.

FIG. 2A is a block diagram illustrating the determination circuit 20 ain detail. As an example, the determination circuit 20 a includes fourpixels 20. The pixel 20 includes the light-receiving circuit 21, theamplifier circuit 22, and a memory circuit 23. The amplifier circuit 22includes an amplifier circuit 22 a, a memory circuit 22 b, and an addercircuit 22 c.

An input terminal of the amplifier circuit 22 a is electricallyconnected to an output terminal of the light-receiving circuit 21 and anoutput terminal of the memory circuit 22 b. An output terminal of theamplifier circuit 22 a is electrically connected to the adder circuit 22c. The light-receiving circuit 21 is electrically connected to thememory circuit 23 through the amplifier circuit 22 a. The memory circuit23 is electrically connected to the signal line OUT1.

The photoelectric conversion element PD converts generated current intovoltage, and the light-receiving circuit 21 outputs the voltage as theoutput signal a. The output signal a is supplied to the amplifiercircuit 22 a. The memory circuit 22 b can set the amplification factorof the amplifier circuit 22 a. The adder circuit 22 c can add an offsetvoltage B to an output signal a1 of the amplifier circuit 22 a. Theadder circuit 22 c outputs the output signal b from its output terminalto an input terminal of the feature extraction circuit 30. Note that theoutput signal a1 may be supplied to the feature extraction circuit 30without the adder circuit 22 c.

The determination circuit 20 a in the block diagram of FIG. 2A functionsas a neuron in the neural network. The neuron functions as a synapse andan activation function. A synapse circuit that functions as a synapsecan multiply each of a plurality of input signals by a weightcoefficient and add the products of the input signals and the weightcoefficient. In other words, the neuron has a function of executing aproduct-sum operation of a plurality of input signals. An activationfunction circuit that functions as an activation function has adetermination function for extracting a feature from the result of aproduct-sum operation.

FIG. 2B represents the block diagram of FIG. 2A as a schematic view of aneuron. A synapse circuit 30N includes the pixels 20 and the featureextraction circuit 30. An activation function circuit 31N includes thedetermination output circuit 31.

FIG. 2B illustrates an example in which four light-receiving circuits 21are connected to the feature extraction circuit 30; however, the numberof connected light-receiving circuits 21 is not limited to four. Forsimplicity, the four light-receiving circuits 21 are denoted by PD(i),PD(i+1), PD(i+2), and PD(i+3) in FIG. 2B. Note that i and j are each anatural number of 1 or more.

The amplifier circuit 22 can multiply the output signal a by a weightcoefficient A. The weight coefficient A is set in the memory circuit 22b in FIG. 2A. The weight coefficient A may be replaced with theamplification factor. Thus, the feature extraction circuit 30 issupplied with the output signal b that is obtained in such a manner thatthe output signal a is multiplied by the weight coefficient A and theoffset voltage B or the like is further added for correction.

The feature extraction circuit 30 can add a plurality of output signalsb. Thus, the output signal c of the feature extraction circuit 30 can beexpressed by the following formula. The weight coefficients A of theamplifier circuits 22 may be the same or different from each other.

Under the above conditions, the total output of the feature extractioncircuit 30 is expressed by the following formula 1.

c(j)=Σ(PD(i)·A(i)+B)  (formula 1)

An output signal c1(j) output from the arithmetic circuit 31 a in theactivation function circuit 31N is expressed by the following formula 2.

c1(j)=ƒ(c(j))  (formula 2)

Note that the output function ƒ(c(j)) of the activation function circuit31N means the sigmoid function. The determination output circuit 31included in the activation function circuit 31N may be supplied with athreshold potential from the outside as a determination condition or afixed threshold potential. Thus, the determination output circuit 31 cancreate a condition called firing in the neural network and output abinarized digital signal.

FIGS. 3A to 3C illustrate examples of the circuits in FIG. 2A. FIG. 3Aillustrates a circuit example of the pixel 20, and FIG. 3B illustrates acircuit example of the determination circuit 20 a. FIG. 3C illustrates acircuit example of the adder circuit 22 c included in the amplifiercircuit 22.

FIG. 3A illustrates the pixel 20 in detail. The pixel 20 includes thelight-receiving circuit 21 and the amplifier circuit 22. The amplifiercircuit 22 includes the amplifier circuit 22 a and the memory circuit 22b. The light-receiving circuit 21 includes the photoelectric conversionelement PD, capacitors C1 and C2, and transistors 41 to 43. Oneelectrode of the photoelectric conversion element PD is electricallyconnected to a terminal VPD 71. The other electrode of the photoelectricconversion element PD is electrically connected to one of a source and adrain of the transistor 41 and one of a source and a drain of thetransistor 42. The other of the source and the drain of the transistor41 is electrically connected to one electrode of the capacitor CA. Agate of the transistor 41 is electrically connected to a terminal Tx 61.The other of the source and the drain of the transistor 42 iselectrically connected to one of a source and a drain of the transistor43 and a terminal VRS 72. A gate of the transistor 42 is electricallyconnected to a gate of the transistor 43 and a terminal RS 62. The otherof the source and the drain of the transistor 43 is electricallyconnected to one electrode of the capacitor C2. The elements in thelight-receiving circuit 21 may be connected to each other in differentmanners, which are illustrated in FIGS. 18A to 18C and FIGS. 19A and19B. FIG. 19B illustrates an example in which the transistor 43 is notprovided.

The capacitor C1 can hold a potential generated by the photoelectricconversion element PD as the output signal a. The capacitor C2 can holda reference potential whose magnitude is to be compared with that of theoutput signal a. The transistors 41 to 43 can control timings of holdingand resetting signals.

The Gilbert cell circuit can be used for the amplifier circuit 22 a. Theamplifier circuit 22 a includes a transistor 44 a, a transistor 45 a, atransistor 44 b, a transistor 45 b, a transistor 46, a transistor 47, atransistor 48, a resistor Ra, and a resistor Rb. One electrode of theresistor Ra is electrically connected to one electrode of the resistorRb and a terminal VPI 73. The other electrode of the resistor Ra iselectrically connected to one of a source and a drain of the transistor44 a and one of a source and a drain of the transistor 45 b. The otherof the source and the drain of the transistor 44 a is electricallyconnected to one of a source and a drain of the transistor 45 a and oneof a source and a drain of the transistor 46. The other of the sourceand the drain of the transistor 45 a is electrically connected to theother electrode of the resistor Rb and one of a source and a drain ofthe transistor 44 b. The other of the source and the drain of thetransistor 44 b is electrically connected to the other of the source andthe drain of the transistor 45 b and one of a source and a drain of thetransistor 47. The other of the source and the drain of the transistor46 is electrically connected to the other of the source and the drain ofthe transistor 47 and one of a source and a drain of the transistor 48.A gate of the transistor 47 is electrically connected to a terminal VCS.A gate of the transistor 48 is electrically connected to a terminalVbias1. The other of the source and the drain of the transistor 48 iselectrically connected to a terminal VSS 79. A gate of the transistor 44a is electrically connected to a gate of the transistor 44 b and the oneelectrode of the capacitor C1. A gate of the transistor 45 a iselectrically connected to a gate of the transistor 45 b and the oneelectrode of the capacitor C2.

The transistors 44 a and 45 a form a differential amplifier circuit, andthe transistors 44 b and 45 b form another differential amplifiercircuit. The differential amplifier circuits each compare the outputsignal a of the capacitor C1 with the referential potential of thecapacitor C2 and perform amplification.

The memory circuit 22 b includes a transistor 49 and a capacitor C3. Oneof a source and a drain of the transistor 49 is electrically connectedto a terminal Wd1 75. The other of the source and the drain of thetransistor 49 is electrically connected to one electrode of thecapacitor C3 and a gate of the transistor 46. A gate of the transistor49 is electrically connected to a terminal W1 74.

In the memory circuit 22 b, a potential is supplied as the amplificationfactor from a signal line Wd1 to the capacitor C3 through the transistor49. The amplification factor is calculated in the outside and suppliedto the memory circuit 22 b. The amplifier circuit 22 amplifies an analogcircuit. Accordingly, the circuit size can be reduced. In addition, theamplifier circuit 22 has the trackablity to the output signal a and anoise smoothing function. Although not illustrated in FIG. 3A, a columndriver or a row driver may be additionally provided to supply theamplification factor to the capacitor C3. Alternatively, the decodercircuit 27 shown in FIG. 1 may be used.

The memory circuit 22 b can control the amplification factor of theamplifier circuit 22 a. The amplification factor corresponds to theweight coefficient A in the synapse circuit. In the case where the sameweight coefficient A is set for the output signals a of all thelight-receiving circuits 21, all the output signals a1 are uniformlyamplified, leading to an improvement in the light-receiving accuracy inlow gray level. In the case where different weight coefficients A areset, the output signals a1 emphasize patterns depending on the weightcoefficients A, facilitating extraction of a specific pattern.

The transistor 48 controls the total amount of current in the amplifiercircuit 22 a. A gate of the transistor 48 is controlled through aterminal Vbias1 in the case where the imaging element 10 is not used orthe imaging element 10 is intentionally not operated. Thus, thetransistor 48 can stop the operation of the amplifier circuit 22,leading to a reduction in power consumption. The use of an oxidesemiconductor in a semiconductor layer of the transistor 48 can reduceits off-state current, in which case the standby current of theamplifier circuit 22 in an off state can be reduced.

The memory circuit 23 can hold an output signal a2 of the amplifiercircuit 22 a. The output signal a2 held in the memory circuit 23 istransferred to the A/D converter circuit 26 through the signal line OUT1when a scan signal is supplied to the scan line G1. The capacitor C1 canhold the output signal a when “Low” is supplied to the terminal Tx 61.Note that the pixel 20 does not necessarily include the memory circuit23.

FIG. 3B illustrates a circuit example of the determination circuit 20 a.Note that the adder circuit 22 c is included in the amplifier circuit22. FIG. 3C illustrates an example of the adder circuit 22 c formedusing passive elements. The adder circuit 22 c includes a plurality ofresistors.

An addition parameter can be supplied as voltage from a terminal Vbias2.The same addition parameter may be supplied to all the pixels in theimaging element 10. Alternatively, different voltages may be supplied tothe pixels, in which case a memory circuit is additionally provided. Theaddition parameter can be used to correct an output of the amplifiercircuit 22 and thus can be used for offset adjustment. The structure ofthe adder circuit 22 c is not limited to the structure illustrated inFIG. 3C as long as it can perform addition to a signal.

Next, the feature extraction circuit 30 and the determination outputcircuit 31 included in the determination circuit 20 a are described. Anexample in which an operational amplifier is used for the adder circuitin the feature extraction circuit 30 is described. The featureextraction circuit 30 includes an operational amplifier 30 a andresistors R1, R2, R3, R4, Rc, and Rf.

One terminal of each of the resistors R1, R2, R3, and R4 is electricallyconnected to the corresponding amplifier circuit 22. The other terminalof each of the resistors R1, R2, R3, and R4 is electrically connected toa negative input terminal of the operational amplifier 30 a. Oneterminal of the resistor Rf is electrically connected to the negativeinput terminal of the operational amplifier 30 a, and the other terminalof the resistor Rf is electrically connected to an output terminal ofthe operational amplifier 30 a. The resistors can each have anappropriate resistivity as needed.

The negative input terminal of the operational amplifier 30 a is areference point at which a virtual short exists, so that the resistor Rfcan perform current-voltage conversion. Accordingly, the result obtainedby addition of the output signals b of the pixels 20 is output to theoutput terminal of the operational amplifier 30 a as a voltage value. Anoutput terminal of the feature extraction circuit 30 is supplied withthe output signal c that is an analog signal.

Next, the determination output circuit 31 is described. Thedetermination output circuit 31 includes the arithmetic circuit 31 a andthe memory circuit 31 b. The arithmetic circuit 31 a may include amemory for holding a voltage of the determination condition.

An input terminal of the arithmetic circuit 31 a is electricallyconnected to the output terminal of the operational amplifier 30 a. Anoutput terminal of the arithmetic circuit 31 a is connected to an inputterminal of the memory circuit 31 b. In the case where the arithmeticcircuit 31 a includes a memory circuit, a column driver and a row driverfor writing a voltage of the determination condition may be additionallyprovided. Alternatively, the decoder circuit 27 shown in FIG. 1 may beused.

The arithmetic circuit 31 a can determine the output signal c of thefeature extraction circuit 30 using the output function ƒ. In the caseof software processing, processing can be performed with the sigmoidfunction or the like. In the case of hardware processing, the sameprocessing can be performed using the arithmetic circuit 31 a.

The amplifier circuit 31 a is supplied with a voltage of thedetermination condition through a signal line Wd2. The arithmeticcircuit 31 a compares the output signal c of the feature extractioncircuit 30 with the voltage of the determination condition. In the casewhere the voltage of the output signal c is higher than the voltage ofthe determination condition, the arithmetic circuit 31 a outputs asignal “High”. In the case where the voltage of the output signal c islower than the voltage of the determination condition, the arithmeticcircuit 31 a outputs a signal “Low”. In this manner, the output signalsa of the plurality of pixels are processed by the neuron and can beconverted into a binarized digital signal using the output function ƒ.

The binarized signal is held in the memory circuit 31 b and can be readas needed. The signal is read with a scan signal supplied to the scanline G2, and output to the selector circuit 28 through the signal lineOUT. For reading of data from the memory circuit 31 b, a column driverand a row driver may be additionally provided. Alternatively, thedecoder circuit 27 may be used.

FIG. 4A is a timing chart of the imaging device 100 in FIG. 1 . A scansignal is supplied to the pixel 20 from the decoder circuit 27 throughthe scan line G1(j), and data held in the memory circuit 23 istransferred to the A/D converter circuit 26. A scan signal is suppliedto the determination circuit 20 a from the decoder circuit 27 throughthe scan line G2(k), and data held in the memory circuit 31 b istransferred to the selector circuit 28. The memory circuits 23 and 31 bcan each change its operation between data acquisition and datatransfer. Thus, the memory circuit 31 b may be formed using a transfergate. The scan signal supplied from the decoder circuit can be used as asignal for controlling the transfer gate.

FIG. 4B is a timing chart of the light-receiving circuit 21 included inthe pixel 20 in FIG. 3A. The operation of the light-receiving circuit 21is controlled through the scan line G1(j). The scan signal suppliedthrough the scan line G1(j) is also supplied to the terminal RS 62. Aperiod from T1 to T2 in FIG. 4A corresponds to a period from T11 to T13in FIG. 4B.

In a period from T11 to T12, a holding potential of the capacitor C2 isrefreshed with a potential supplied to the terminal VRS 72. In theperiod from T11 to T12, “Low” is supplied to the terminal Tx 61 and thetransistor 41 remains off. The transistor 41 remains off for a period oftime needed for data to be stored in the memory circuit 23 and thememory circuit 31 b. Since the memory circuit 23 and the memory circuit31 b may each be formed using a transfer gate, the period from T11 toT12 can be shortened by using a transistor with high mobility. Thetransistor 41 is preferably a transistor including a CAC-OS to bedescribed in Embodiment 6.

In a period from T12 to T13, the terminal Tx 61 is set to “High”, andthe transistors 41 to 43 are turned on. Accordingly, the holdingpotential of the capacitor C1 is refreshed with a voltage supplied tothe terminal VRS 72.

At T13, the scan signal supplied to the scan line G1(j) is set to “Low”.The terminal RS 62 is also set to “Low”. Consequently, the transistors42 and 43 are turned off and the transistor 41 remains on. Accordingly,the photoelectric conversion element PD acquires data. Data is acquireduntil the scan line G1(j) is selected in the next frame. Note that thedecoder circuit included in the imaging device 100 may be divided into aplurality of regions and may perform parallel processing. In this case,the data acquisition period can be shorter than one frame.

The imaging element 10, which includes the determination circuits 20 a,can perform analog arithmetic processing using analog data in a mannersimilar to analog data processing of a neuron in the brain. The imagingelement 10 can perform arithmetic processing while the frequency ofconversion of analog data into digital data is kept as low as possible.

The neural network requires an enormous amount of arithmetic processingand hierarchical processing. However, with the structure in thisembodiment, the determination circuit 20 a can perform processingcorresponding to processing in an input layer of the multilayerperceptron in the neural network. Accordingly, the determination circuit20 a corresponding to the input layer can obtain two kinds of outputresult, i.e., a signal of data of received light and a signal obtainedby analog arithmetic processing using the data of received light in thepixels 20. Thus, in the imaging device, the amount of arithmeticprocessing by software can be reduced and power consumption of thearithmetic processing can be reduced. Moreover, time required for thearithmetic processing can be shortened.

In this embodiment, the imaging device 100 can output normal image dataand data compatible with the neural network. Since dealing withdifferent kinds of data, the imaging device 100 preferably performsprocessing in synchronization with a frame. The timing of imaging issynchronized with the selection order of scan lines, so that a time lagis caused. Therefore, a global shutter system is preferably employedwhen the imaging device 100 takes an image of a fast moving object.

In the global shutter system, it is preferable that the terminals Tx 61and the terminals RS 62 of all the light-receiving circuits 21 includedin the imaging element 10 be controlled simultaneously. In this manner,the imaging device 100 can simultaneously acquire all the data ofreceived light of the light-receiving circuits 21. The memory circuits31 b included in the determination circuits 20 a are simultaneouslysupplied with data processed arithmetically by the multi-layerperceptron.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 2

In this embodiment, an imaging device that includes an interfaceconnected to a neural network is described with reference to FIG. 5 ,FIGS. 6A and 6B, FIG. 7 , and FIGS. 8A and 8B.

One embodiment of the present invention is a structure and an operatingmethod of an imaging device different from that in Embodiment 1.

FIG. 5 is a block diagram illustrating a structure example of theimaging device 100 different from that in FIG. 1 . The determinationcircuit 20 a having a structure different from the structure illustratedin FIG. 1 is described. The determination circuit 20 a in FIG. 5 isdifferent from that in FIG. 1 in including an amplifier circuit 300 anda determination output circuit 310.

Four pixels 20 included in the determination circuit 20 a is describedwith reference to FIG. 5 .

The pixels 20 are electrically connected to the amplifier circuit 300.The amplifier circuit 300 is electrically connected to the determinationoutput circuit 310.

Note that the number of pixels 20 included in the determination circuit20 a is preferably determined as appropriate in accordance with a regionto be determined. A light-receiving circuit 21 a in the pixel 20 may beconnected to a plurality of amplifier circuits 300.

The photoelectric conversion element PD included in the light-receivingcircuit 21 a in the pixel 20 can convert received light into voltage.The pixel 20 outputs an output signal a. The amplifier circuit 300 canconvert an analog signal to a digital signal and output an output signalb that is an amplified digital signal.

The output signal b is subjected to arithmetic operation in thedetermination output circuit 310. Addition or multiplication ispreferably performed as the arithmetic operation. In this embodiment,the determination output circuit 310 is an adder circuit.

The determination output circuit 310 can extract a feature of data fromthe output signal b. The extracted data is determined and thedetermination result can be output as an output signal d.

FIG. 6A is a block diagram illustrating the determination circuit 20 ain detail. As in FIG. 5 , the determination circuit 20 a includes thefour pixels 20, for example. Each of the pixels 20 includes thelight-receiving circuit 21 a and the memory circuit 23. Thedetermination circuit 20 a includes the amplifier circuit 300, a featureextraction circuit 32, and an output circuit 33.

The amplifier circuit 300 includes an input selection circuit 301, anA/D converter circuit 302, a determination circuit 303, and a memorycircuit 304. The determination circuit 303 includes logic circuits 306and a selection circuit 305.

The light-receiving circuit 21 a is electrically connected to the memorycircuit 23. The memory circuit 23 is electrically connected to thesignal line OUT1. An output terminal of the light-receiving circuit 21 ais electrically connected to an input terminal of the input selectioncircuit 301 included in the amplifier circuit 300.

The photoelectric conversion element PD included in the light-receivingcircuit 21 a converts generated current into voltage, and thelight-receiving circuit 21 a can output the voltage as the output signala. The output signal a can be supplied to an input terminal of theamplifier circuit 300.

The input selection circuit 301 is electrically connected to the A/Dconverter circuit 302. The A/D converter circuit 302 is electricallyconnected to the determination circuit 303. The determination circuit303 is electrically connected to the memory circuit 304.

The input selection circuit 301 can select one of the four outputsignals a by using a signal generated from a clock signal applied to aterminal CLK. The A/D converter circuit 302 can convert the selectedoutput signal a, which is voltage, into a digital signal and output thedigital signal to an input terminal of the determination circuit 303.The determination circuit 303 can amplify a digital signal by bit shift.A high-order bit is extracted by bit shift, and thus the level ofhigh-order bit can be determined. The determination result of thehigh-order bit can be held in the memory circuit 304. The held signalcan be supplied to an input terminal of the feature extraction circuit32 as the output signal b.

The feature extraction circuit 32 is electrically connected to theoutput circuit 33. The feature extraction circuit 32 extracts a featureof data from the output signal b supplied to its input terminal. Theextracted data is counted as a count value and supplied to an inputterminal of the output circuit 33 as an output signal c. The outputsignal c is determined by the output circuit 33, and the determinationresult can be output as the output signal d.

The determination circuit 20 a in the block diagram of FIG. 6A functionsas a neuron in the neural network. The neuron functions as a synapse andan activation function. A synapse circuit that functions as a synapsecan multiply each of a plurality of input signals by a weightcoefficient and add the products of the input signals and the weightcoefficient. In other words, the neuron has a function of executing aproduct-sum operation of a plurality of input signals. An activationfunction circuit that functions as an activation function has adetermination function for extracting a feature from the result of aproduct-sum operation.

FIG. 6B represents the block diagram of FIG. 6A as a schematic view of aneuron. A synapse circuit 32N includes the amplifier circuit 300 and thefeature extraction circuit 32. An activation function circuit 33Nincludes the output circuit 33.

FIG. 6B illustrates an example in which four light-receiving circuits 21a are connected to the amplifier circuit 300; however, the number ofconnected light-receiving circuits 21 a is not limited to four. Forsimplicity, the four light-receiving circuits 21 a are denoted by PD(i),PD(i+1), PD(i+2), and PD(i+3) in FIG. 6B. Note that i and j are each anatural number of 1 or more.

The amplifier circuit 300 can multiply the output signal a by the weightcoefficient A. The weight coefficient A is set in the determinationcircuit 303 in FIG. 6A. The weight coefficient A may be replaced withthe amplification factor. Thus, the feature extraction circuit 32 issupplied with data that is obtained in such a manner that the outputsignal a is multiplied by the weight coefficient A, as the output signalb.

Note that the weight coefficients A of the determination circuit 303 maybe the same or different from each other.

Under the above conditions, the total output of the feature extractioncircuit 32 can be expressed by the formula 1 in Embodiment 1. Theformula 2 in Embodiment 1 can be used to obtain an output signal d(i)output from the activation function circuit 33N.

The output function ƒ(c(i) of the activation function circuit 33N meansthe sigmoid function. In the output circuit 33 included in theactivation function circuit 33N, the determination condition may beupdated or fixed. Thus, a condition called firing in the neural networkcan be created using the output circuit 33 and a binarized digitalsignal can be output.

FIG. 7 illustrates examples of the circuits in FIG. 6A. FIG. 7illustrates circuit examples of the pixel 20, the amplifier circuit 300,the feature extraction circuit 32, and the output circuit 33.

First, the pixel 20 is described. The pixel 20 includes thelight-receiving circuit 21 a and the memory circuit 23. Thelight-receiving circuit 21 a includes the photoelectric conversionelement PD, the capacitor C1, the transistor 41, and the transistor 42.

The capacitor C1 can hold a potential generated by the photoelectricconversion element PD as the output signal a. The transistors 41 and 42can control timings of holding and resetting signals.

The output signal a is held in the memory circuit 23 and can be read asneeded. The signal is read with a scan signal supplied to the scan lineG1, and transferred to the A/D converter circuit 26 through the signalline OUT1. A column driver or a row driver for reading a signal from thememory circuit 23 may be additionally provided. Alternatively, thedecoder circuit 27 may be used.

Next, the amplifier circuit 300 is described. The amplifier circuit 300includes the input selection circuit 301, the A/D converter circuit 302,the memory circuit 304, the selection circuit 305, the logic circuits306, and a counter circuit CN1. Assume that the four light-receivingcircuits 21 a are electrically connected to the amplifier circuit 300.The clock signal is supplied to the amplifier circuit 300 from theterminal CLK. The clock signal, which is a reference for a circuitoperation, is also supplied to the determination output circuit 310.

The input selection circuit 301 can select one of the four outputsignals a. In one of selection methods, the counter circuit CN1 can beused. The counter circuit CN1 can supply an output signal cnt1 to theinput selection circuit 301. The counter circuit CN1 can have a sizedepending on the number of light-receiving circuits 21 a connected tothe amplifier circuit 300. The counter circuit CN1 performs a countoperation in synchronization with the clock signal supplied to theterminal CLK; therefore, the input selection circuit 301 cansequentially select the output signal a in synchronization with theclock signal.

The input selection circuit 301 can supply the output signal a selectedin accordance with the output signal cnt1 to the A/D converter circuit302. As an example, the A/D converter circuit 302 converts the outputsignal a, which is voltage, into an 8-bit digital signal D[7:0]. The A/Dconverter circuit 302 preferably select the data width as necessary.

As a method for amplifying a digital signal, a method in whicharithmetic operation is performed in such a way that a bit is shiftedout by bit shift is employed. In the determination circuit 303, thedigital signal D[7:0] is amplified by bit shift, and the level of thedigital signal can be classified into one of a plurality of ranges.

In bit shift, bit shift(s) to the left can amplify a value by a power oftwo (e.g., double, quadruple, or eight times). Thus, the case where thehighest-order bit D[7] of the digital signal D[7:0] is “High” by oneleft bit-shift means that the digital signal is greater than 128 LSB.The case where the two highest-order bits D[7:6] are “High” means thatthe digital signal is greater than 192 LSB. In this manner, theselection circuit 305 can amplify a digital signal and the level of thedigital signal can be classified into one of a plurality of ranges.

A signal for selecting the range of level of the digital signal issupplied to the selection circuit 305 from a terminal GAIN. Theselection circuit 305 can supply, to the memory circuit 304, a signal“High” in the case where the digital signal is within the specifiedselection range and a signal “Low” in the case where the digital signalis not within the specified range. The signal output to the memorycircuit 304 is referred to as an output signal a1.

Note that the amount of bit shift, which is the amplification factor, iscalculated in the outside and supplied from the terminal GAIN.Therefore, in the amplifier circuit 300, all the determinations may beperformed under the same condition or different conditions. Thedetermination conditions of the logic circuits 306 may be reconfiguredusing a programmable logic array depending on processing.

A latch circuit can be used for the memory circuit 304. The use of alatch circuit for the memory circuit 304 can reduce the circuit size andthe number of signals to be controlled, which is preferable. The timingof writing to the memory circuit 304 can be determined in accordancewith the output signal cnt1 of the counter circuit CN1. For example, inthe case where a period during which the output signal cnt1 is “High” isa selection period of the input selection circuit 301, the output signala1 is held in the memory circuit 304 in synchronization with the timingof the fall of the output signal cnt1. The held signal is supplied tothe input terminal of the feature extraction circuit 32 as the outputsignal b.

The feature extraction circuit 32 can extract a feature of data from theoutput signal b. The feature indicates whether the output signal aextracted by the determination circuit 303 is within the specifiedrange.

The feature extraction circuit 32 includes an input selection circuit 32a, a counter circuit 32 c, a counter circuit CN2, and an inverter 32 b.The output circuit 33 includes a determination circuit 33 a, a switchcircuit 33 b, and a memory circuit 33 c.

The input selection circuit 32 a can select one of four output signalsb. As a method for selection, for example, a method in which the countercircuit CN2 is used can be employed. The counter circuit CN2 takes indata after the output signal b of the memory circuit 304 is determined,and thus the clock signal supplied to the amplifier circuit 300 can beinverted by the inverter 32 b to be supplied to the counter circuit CN2.The input selection circuit 32 a can sequentially output the outputsignals b as output signals b1.

The counter circuit 32 c can count the number of output signals b1 thatare within the specified range supplied from the terminal GAIN. In thecase where the level of the output signal a is within the specifiedrange, the output signal b1 is “High”, whereas in the case where thelevel is not within the range, the output signal b1 is “Low”.

Thus, the counter circuit 32 c can count the number of signals includingthe feature of data. The count result can be supplied to the outputcircuit 33 of the determination output circuit 310 as the output signalc.

A determination value is supplied to the determination circuit 33 a froma terminal CMPD. The determination circuit 33 a determines whether theoutput signal c including the feature of data is higher than thedetermination value or not. An output signal dout is supplied to thememory circuit 33 c as a determination result. Note that the outputsignal c may be directly supplied to the memory circuit 33 c. The switchcircuit 33 b can change the determination methods.

Although a variety of memory circuits can be used as the memory circuit33 c, a circuit that can make output to have high impedance ispreferably used. For example, a memory including a transistor in whichsilicon is included in a semiconductor layer can be used. Alternatively,a memory including a transistor in which an oxide semiconductor isincluded in a semiconductor layer may be used. Note that the details ofthe oxide semiconductor will be described in Embodiment 6.

It is preferable that the output signal dout held in the memory circuit33 c be read as needed. The signal is read with a scan signal suppliedto the scan line G2 and transferred to the selector circuit 28 throughthe signal line OUT. For reading data from the memory circuit 33 c, acolumn driver and a row driver may be additionally provided.Alternatively, the decoder circuit 27 in FIG. 1 may be used.

FIG. 8A is a timing chart of the imaging device 100 in FIG. 5 . In aperiod from T21 to T22, a scan signal is supplied to the pixel 20 fromthe decoder circuit 27 through the scan line G1 (j), and data held inthe memory circuit 23 is transferred to the A/D converter circuit 26. Ascan signal is supplied to the determination circuit 20 a from thedecoder circuit 27 through the scan line G2(k), and data held in thememory circuit 33 c is transferred to the selector circuit 28.

FIG. 8B is a timing chart of the determination circuit 20 a in FIG. 7 .The operation of the light-receiving circuit 21 a is controlled throughthe scan line G1(j). The scan signal supplied through the scan lineG1(j) is also supplied to the terminal RS 62. The period from T21 to T22in FIG. 8A correspond to a period from T31 to T43 in FIG. 8B.

In a period from T31 to T41, “Low” is supplied to the terminal Tx 61,and the transistor 41 remains off. In addition, “High” is supplied tothe terminal RS 62, and the transistor 42 remains on. Accordingly, theholding potential of the capacitor C1 is supplied to the amplifiercircuit 300 as the output signal a. The output signal a is subjected toarithmetic processing in the amplifier circuit 300 and the determinationoutput circuit 310.

In a period from T41 to T43, the terminal Tx 61 and the terminal RS 62are set to “High”, and the transistors 41 and 42 are turned on.Accordingly, the holding potential of the capacitor C1 is refreshed witha voltage supplied to the terminal VRS 72.

At T43, the scan signal supplied to the scan line G1(j) is set to “Low”.The terminal RS 62 is also set to “Low”. Consequently, the transistor 42is turned off and the transistor 41 remains on. Accordingly, thephotoelectric conversion element PD acquires data. Data is acquireduntil the scan line G1(j) is selected in the next frame. Note that thedecoder circuit included in the imaging device 100 may be divided into aplurality of regions and may perform parallel processing. In this case,the data acquisition period can be shorter than one frame.

FIG. 8B shows an example in which the scan signal supplied to the scanline G1(j) and the scan signal supplied to the scan line G2(k−1) aresupplied at the same timing. Note that the timing of reading data fromthe memory circuit 33 c may be independently controlled with the scansignal supplied to the scan line G2.

The imaging element 10 illustrated in FIG. 7 , which includes thedetermination circuits 20 a, can convert analog data into digital dataand perform arithmetic processing in a manner similar to processingexecuted by a neuron in the brain. With the above structure, the imagingelement 10 can extract a feature of data from analog data and performcompression arithmetic processing on digital data. Thus, the imagingelement 10 can perform parallel multiprocessing.

The neural network requires an enormous amount of arithmetic processingand hierarchical processing. However, with the structure in thisembodiment, the determination circuit 20 a can perform processingcorresponding to processing in an input layer of the multilayerperceptron in the neural network. Accordingly, the determination circuit20 a corresponding to the input layer can obtain two kinds of outputresult, i.e., a signal of data of received light and a signal obtainedby digital arithmetic processing using the data of received light in thepixels 20. Thus, in the imaging device, the amount of arithmeticprocessing by software can be reduced and power consumption of thearithmetic processing can be reduced. Moreover, time required for thearithmetic processing can be shortened.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 3

In this embodiment, the structures of the imaging device in Embodiment 1are described with reference to FIG. 9 , FIG. 10 , FIGS. 11A to 11E,FIGS. 12A to 12D, FIG. 13 , FIGS. 14A to 14C, FIG. 15 , FIGS. 16A and16B, FIG. 17 , FIGS. 18A to 18C, FIGS. 19A and 19B, and FIGS. 20A and20B.

FIG. 9 illustrates an example of a specific structure of the pixel 20and is a cross-sectional view in the channel length direction of thetransistors 41 and 42 included in the light-receiving circuit 21 and thetransistors 46 and 47 included in the amplifier circuit 22 a.

Although wirings, electrodes, metal layers, and contact plugs(conductors 82) are shown as independent components in cross-sectionalviews in this embodiment, some of them are provided as one component insome cases when they are electrically connected to each other. Inaddition, a structure in which components such as wirings, electrodes,and metal layers are connected to each other through the conductors 82is only an example, and the components may be directly connected to eachother not through the conductor 82.

As illustrated in FIG. 9 , FIG. 10 , FIGS. 11A to 11E, FIGS. 12A to 12D,FIG. 13 , FIGS. 14A to 14C, FIG. 15 , and FIG. 17 , insulating layers 81a to 81 g, 81 j, and the like that function as protective films,interlayer insulating films, or planarization films are provided over asubstrate and components such as transistors. For example, an inorganicinsulating film such as a silicon oxide film or a silicon oxynitridefilm can be used as the insulating layers 81 a to 81 g. Alternatively,an organic insulating film such as an acrylic resin film or a polyimideresin film may be used. Top surfaces of the insulating layers 81 a to 81g and the like may be subjected to planarization treatment as necessaryby chemical mechanical polishing (CMP) or the like.

In some cases, some of wirings, transistors, and the like illustrated indrawings are not provided, or a wiring, a transistor, or the like thatis not illustrated in drawings is included in each layer.

As illustrated in FIG. 9 , the pixel 20 can include a layer 1100 and alayer 1200.

The layer 1100 can include the photoelectric conversion element PD. Aphotodiode with two terminals can be used for the photoelectricconversion element PD, for example. The photodiode can be a PNphotodiode using a single crystal silicon substrate, a PIN photodiodeusing an amorphous silicon thin film, a microcrystalline silicon thinfilm, or a polycrystalline silicon thin film, or a photodiode usingselenium, a selenium compound, or an organic compound, for example.

In FIG. 9 , the photoelectric conversion element PD included in thelayer 1100 is a PN photodiode using a single crystal silicon substrate.The photoelectric conversion element PD can include the insulating layer81 j, a p⁺ region 620, a p⁻ region 630, an n-type region 640, and a p⁺region 650.

In the layer 1200, the transistors included in the light-receivingcircuit 21, the amplifier circuit 22 a, and the memory circuit 22 b caninclude an oxide semiconductor in their semiconductor layers. In FIG. 9, the transistors 41 and 42 included in the light-receiving circuit 21and the transistors 46 and 47 included in the amplifier circuit 22 a areillustrated as examples.

As shown in FIG. 9 , the photoelectric conversion element PD can overlapwith the light-receiving circuit 21 and the amplifier circuit 22 a;thus, the area where the photoelectric conversion element PD receiveslight can be widened. Note that an example of the oxide semiconductorwill be described in detail in Embodiment 6.

An insulating layer 80 is provided between a region including OStransistors and a region including Si devices (e.g., Si transistors orSi photodiodes).

An insulating layer provided near a Si device preferably containshydrogen to terminate dangling bonds of silicon. Meanwhile, hydrogen ininsulating layers provided near oxide semiconductor layers that aresemiconductor layers of the transistors 41, 42, and the like causesgeneration of carriers in the oxide semiconductor layers. Thus, thehydrogen might reduce the reliability of the transistors 41, 42, and thelike. For this reason, the insulating layer 80 that has a function ofpreventing diffusion of hydrogen is preferably provided between onelayer including the Si device and another layer that includes the OStransistors and is stacked over the one layer. Diffusion of hydrogen canbe prevented by the insulating layer 80; thus, reliability of both theSi device and the OS transistors can be improved.

The insulating layer 80 can be formed using aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, hafnium oxynitride, or yttria-stabilizedzirconia (YSZ), for example.

One electrode (the n-type region 640) of the photoelectric conversionelement PD can be electrically connected to the transistor 41 and thetransistor 42 through two conductors 82 and a wiring 69, for example.

Here, the conductor 82 is provided to penetrate the insulating layer 80;hence, the conductor 82 also preferably has a function of preventingdiffusion of hydrogen. For example, the conductor 82 has the followingstructure illustrated in FIG. 9 : a conductor 82 b with a barrierproperty against hydrogen is provided at least on the outer side incontact with a sidewall of an opening, and a conductor 82 a with lowresistance is provided on the inner side. For instance, tungsten can beused for the conductor 82 a, and tantalum nitride or the like can beused for the conductor 82 b. Note that the conductor 82 can also beformed only of the conductor 82 a. When a layer containing impuritiessuch as hydrogen is not in contact with the conductor 82, the conductor82 may be formed only of the conductor 82 b.

In FIG. 9 , top-gate OS transistors are provided in the layer 1200. Forexample, each of the OS transistors is provided over a stack includinginsulating layers (the insulating layers 81 a, 80, and 81 b) formed overthe layer 1100, and includes an oxide semiconductor layer 130,conductive layers 140 and 150 serving as a source electrode and a drainelectrode, an insulating layer 160 serving as a gate insulating layer,and a conductive layer 170 serving as a gate electrode. Note that theinsulating layer 81 b can also function as a gate insulating layer.

FIG. 9 illustrates an example of a structure where the OS transistorsare each provided with a conductive layer 173 serving as a back gateelectrode. In the structure of FIG. 9 , back gate electrodes that doubleas light-blocking layers are preferably provided because light thatpasses through the layer 1100 might change the electricalcharacteristics of the transistors. Moreover, providing the back gatesenables control of the threshold voltages and the like of the OStransistors.

Alternatively, the pixel 20 can employ a stacked structure illustratedin FIG. 10 . In the pixel 20 in FIG. 10 , the layer 1200 and the layer1100 are provided over a substrate 115. The photoelectric conversionelement PD is provided over the OS transistors, which facilitateselectrical connection between the OS transistors and one electrode ofthe photoelectric conversion element PD.

FIG. 10 illustrates the photoelectric conversion element PD using aselenium-based material for a photoelectric conversion layer 561. Thephotoelectric conversion element PD containing a selenium-based materialhas high external quantum efficiency with respect to visible light.Furthermore, a selenium-based material has a high light-absorptioncoefficient, making the photoelectric conversion layer 561 thin easily.The photoelectric conversion element PD containing a selenium-basedmaterial can be a highly sensitive sensor in which the amount ofamplification of signals is large because of avalanche multiplication.In other words, the use of a selenium-based material for thephotoelectric conversion layer 561 allows a sufficient amount ofphotocurrent to be obtained even when the pixel area is reduced. Thus,the photoelectric conversion element PD containing a selenium-basedmaterial is also suitable for imaging in a low-illuminance environment.

As a selenium-based material, amorphous selenium or crystalline seleniumcan be used. Crystalline selenium can be obtained, for example, bydepositing amorphous selenium and then performing heat treatment. Whenthe crystal grain size of crystalline selenium is smaller than a pixelpitch, variations in characteristics between pixels can be reduced.Moreover, crystalline selenium has higher spectral sensitivity andlight-absorption coefficient for visible light than amorphous selenium.

Although the photoelectric conversion layer 561 is a single layer inFIG. 10 , a layer of gallium oxide, cerium oxide, In—Ga—Zn oxide, or thelike may be provided as a hole-injection blocking layer 568 on alight-receiving surface side as illustrated in FIG. 11A. Alternatively,as illustrated in FIG. 11B, a layer of nickel oxide, antimony sulfide,or the like may be provided as an electron-injection blocking layer 569on an electrode 566 side. Further alternatively, the hole-injectionblocking layer 568 and the electron-injection blocking layer 569 may beprovided as illustrated in FIG. 11C.

The photoelectric conversion layer 561 may be a layer containing acompound of copper, indium, and selenium (CIS); or a layer containing acompound of copper, indium, gallium, and selenium (CIGS). The use of CISor CIGS makes it possible to form a photoelectric conversion elementthat utilizes avalanche multiplication as in the case of using a singlelayer of selenium.

In the photoelectric conversion element PD using a selenium-basedmaterial, for example, the photoelectric conversion layer 561 can beprovided between a light-transmitting conductive layer 562 and theelectrode 566 formed using a metal material or the like. Furthermore,CIS and CIGS are p-type semiconductors, and an n-type semiconductor suchas cadmium sulfide or zinc sulfide may be provided in contact with thep-type semiconductor in order to form a junction.

Although the light-transmitting conductive layer 562 is directly incontact with a wiring 571 in FIG. 10 , they may be in contact with eachother through a wiring 588 as illustrated in FIG. 11D. Although thephotoelectric conversion layer 561 and the light-transmitting conductivelayer 562 are not divided between pixels in FIG. 10 , they may bedivided between circuits as illustrated in FIG. 11E. In a region betweenpixels where the electrode 566 is not provided, a partition wall 567formed of an insulator is preferably provided, thereby preventinggeneration of a crack in the photoelectric conversion layer 561 and thelight-transmitting conductive layer 562. However, the partition wall 567is not necessarily provided as illustrated in FIGS. 12A and 12B.

The electrode 566, the wiring 571, and the like may be a multilayer. Forexample, as illustrated in FIG. 12C, the electrode 566 can include twoconductive layers 566 a and 566 b and the wiring 571 can include twoconductive layers 571 a and 571 b. In the structure in FIG. 12C, it ispreferred that, for example, the conductive layers 566 a and 571 a bemade of a low-resistance metal or the like, and the conductive layers566 b and 571 b be made of a metal or the like that exhibits anexcellent contact property with the photoelectric conversion layer 561.Such a structure improves the electrical properties of the photoelectricconversion element PD. Note that some kinds of metal may causeelectrochemical corrosion by being in contact with thelight-transmitting conductive layer 562; even when such a metal is usedfor the conductive layer 571 a, electrochemical corrosion can beprevented by the conductive layer 571 lb.

The conductive layers 566 b and 571 b can be formed using molybdenum ortungsten, for example. The conductive layers 566 a and 571 a can beformed using, for example, aluminum, titanium, or a stack of titanium,aluminum, and titanium that are stacked in this order.

As illustrated in FIG. 12D, the light-transmitting conductive layer 562may be connected to the wiring 571 through the conductor 82 and thewiring 588.

The partition wall 567 can be formed using an inorganic insulator, aninsulating organic resin, or the like. The partition wall 567 may becolored black or the like in order to shield the transistors and thelike from light and/or to determine the area of a light-receivingportion in each pixel.

Alternatively, the pixel 20 can employ a stacked structure illustratedin FIG. 13 . The pixel 20 in FIG. 13 is different from the pixel 20 inFIG. 10 only in the structure of the layer 1100; the other structuresare the same.

In FIG. 13 , the photoelectric conversion element PD included in thelayer 1100 is a PIN photodiode using an amorphous silicon film, amicrocrystalline silicon film, or the like as a photoelectric conversionlayer. The photoelectric conversion element PD can include an n-typesemiconductor layer 565, an i-type semiconductor layer 564, a p-typesemiconductor layer 563, the electrode 566, the wiring 571, and thewiring 588.

The electrode 566 is in contact with the insulating layer 80. The p-typesemiconductor layer 563 is electrically connected to the electrode 566through the wiring 588. The wiring 588 is provided to penetrate theinsulating layer 81 e.

The i-type semiconductor layer 564 is preferably formed using amorphoussilicon. The p-type semiconductor layer 563 and the n-type semiconductorlayer 565 can each be formed using amorphous silicon, microcrystallinesilicon, or the like that includes a dopant imparting the correspondingconductivity type. A photodiode in which a photoelectric conversionlayer is formed using amorphous silicon has high sensitivity in avisible light wavelength region, and thus can easily sense weak visiblelight.

FIGS. 14A to 14C show other examples of the structure of thephotoelectric conversion element PD having a structure of a PIN thinfilm photodiode and the connection between the photoelectric conversionelement PD and the wirings. Note that the structure of the photoelectricconversion element PD and the connection between the photoelectricconversion element PD and the wirings are not limited thereto, and otherstructures may be employed.

In FIG. 14A, the photoelectric conversion element PD includes thelight-transmitting conductive layer 562 in contact with the p-typesemiconductor layer 563. The light-transmitting conductive layer 562serves as an electrode and can increase the output current of thephotoelectric conversion element PD.

The light-transmitting conductive layer 562 can be formed using, forexample, indium tin oxide, indium tin oxide containing silicon, indiumoxide containing zinc, zinc oxide, zinc oxide containing gallium, zincoxide containing aluminum, tin oxide, tin oxide containing fluorine, tinoxide containing antimony, graphene, or graphene oxide. Thelight-transmitting conductive layer 562 is not limited to a single layerand may be a stack of different films.

In FIG. 14B, the light-transmitting conductive layer 562 and the wiring571 are connected to each other through the conductor 82 and the wiring588. Note that the p-type semiconductor layer 563 of the photoelectricconversion element PD and the wiring 571 can be connected to each otherthrough the conductor 82 and the wiring 588. In the structure of FIG.14B, the light-transmitting conductive layer 562 is not necessarilyprovided.

In FIG. 14C, an opening that exposes the p-type semiconductor layer 563is provided in an insulating layer 81 e covering the photoelectricconversion element PD, and the light-transmitting conductive layer 562covering the opening is electrically connected to the wiring 571.

The photoelectric conversion element PD containing the selenium-basedmaterial, amorphous silicon, or the like can be manufactured throughgeneral semiconductor manufacturing processes such as a depositionprocess, a lithography process, and an etching process. Since theresistance of the selenium-based material is high, the photoelectricconversion layer 561 does not need to be divided between circuits asillustrated in FIG. 10 . Thus, the photoelectric conversion element PDcan be manufactured with a high yield at low cost.

Alternatively, the pixel 20 may have a stacked structure illustrated inFIG. 15 . In the pixel 20 in FIG. 15 , the layer 1200 and the layer 1100are provided over a layer 1300. In the layer 1300, for example, theproduct-sum circuit, the adder circuit, the memory circuit such as alatch, the data converter circuit such as an A/D converter circuit, thebuffer circuit, and the control circuit for the entire imaging device,which are illustrated in FIGS. 3A to 3C, can be provided.

The layer 1300 can include Si transistors used in the amplifier circuit22 a, the memory circuit 22 b, the feature extraction circuit 30, andthe determination output circuit 31 (e.g., the transistors 44 to 48included in the amplifier circuit 22 a). Although FIG. 15 illustrates anexample of FIN-type transistors 44 to 48 provided on a silicon substrate600, the transistors 44 a, 44 b, 45 a, and 45 b may be planartransistors as illustrated in FIG. 16A. Alternatively, as illustrated inFIG. 16B, transistors each including a semiconductor layer 660 formedusing a silicon thin film may be used. The semiconductor layer 660 canbe formed using polycrystalline silicon or single crystal silicon of asilicon-on-insulator (SOI) structure.

FIG. 15 illustrates the structure obtained by adding the layer 1300 tothe structure shown in FIG. 10 ; it is also possible to add the layer1300 to the structure shown in FIG. 13 .

FIG. 17 is a cross-sectional view of a structure obtained by adding alayer 1400 to the structure shown in FIG. 9 , and illustrates threepixels (pixels 20A, 20B, and 20C).

In the layer 1400, a light-blocking layer 1530, optical conversionlayers 1550 a, 1550 b, and 1550 c, a microlens array 1540, and the likecan be provided.

An insulating layer 81 h is formed in a region in contact with the layer1100. As the insulating layer 81 h, a silicon oxide film with a highvisible-light transmitting property can be used, for example. Inaddition, a silicon nitride film may be stacked as a passivation film. Adielectric film of hafnium oxide or the like may be stacked as ananti-reflection film.

The light-blocking layer 1530 can be provided on the insulating layer 81h. The light-blocking layer 1530 is provided at a boundary betweenadjacent pixels and has a function of blocking stray light that entersfrom oblique directions. The light-blocking layer 1530 can be formed ofa metal layer of aluminum, tungsten, or the like, or a stack includingthe metal layer and a dielectric film functioning as an anti-reflectionfilm.

The optical conversion layers 1550 a to 1550 c can be provided on theinsulating layer 81 h and the light-blocking layer 1530. Color imagesare obtained, for example, when color filters of red (R), green (G),blue (B), yellow (Y), cyan (C), magenta (M), and/or the like areassigned to the optical conversion layers 1550 a to 1550 c.

Note that when a filter that blocks light with a wavelength shorter thanor equal to that of visible light is used as the optical conversionlayer, an infrared imaging device is obtained. When a filter that blockslight with a wavelength shorter than or equal to that of near infraredlight is used as the optical conversion layer, a far-infrared imagingdevice is obtained. When a filter that blocks light with a wavelengthlonger than or equal to that of visible light is used as the opticalconversion layer, an ultraviolet imaging device is obtained.

When a scintillator is used as the optical conversion layer, it ispossible to obtain an imaging device that takes an image visualizing theintensity of radiation and is used for an X-ray imaging device or thelike. Radiations such as X-rays that pass through an object to enter ascintillator are converted into light (fluorescence) such as visiblelight or ultraviolet light owing to photoluminescence. Then, thephotoelectric conversion element PD detects the light to obtain imagedata. Moreover, the imaging device having the above structure may beused in a radiation detector or the like.

A scintillator contains a substance that, when irradiated with radiationsuch as X-rays or gamma rays, absorbs energy of the radiation to emitvisible light or ultraviolet light. For example, it is possible to use aresin or ceramics in which any of Gd₂O₂S:Tb, Gd₂O₂S:Pr, Gd₂O₂S:Eu,BaFCl:Eu, NaI, CsI, CaF₂, BaF₂, CeF₃, LiF, LiI, and ZnO is dispersed.

The microlens array 1540 can be provided on the optical conversionlayers 1550 a to 1550 c. Light that passes through lenses of themicrolens array 1540 passes the optical conversion layers 1550 a to 1550c that are placed directly on the microlens array 1540, and is appliedto the photoelectric conversion element PD.

As illustrated in FIG. 18A, the light-receiving circuit 21 may have astructure including floating nodes Fn1 and Fn2 without the capacitors C1and C2. The floating node Fn1 holds charge using the gate capacitancesof the transistors 41 and 44 a and the parasitic capacitance betweenwirings.

The transistors 41 to 43 included in the light-receiving circuit 21 mayeach have a structure with a back gate as illustrated in FIG. 18B. FIG.18B illustrates a structure in which a constant potential is applied tothe back gates, which enables control of the threshold voltages.Although an example in which transistors with back gates are used forthe transistors 41 to 43 is shown, all or some of the transistors usedin the image device 100 may have back gates.

As illustrated in FIG. 18B, wirings connected to the back gates of thetransistors 41 to 43 may be electrically connected to the gates of thecorresponding transistors.

In an n-channel transistor, the threshold voltage is shifted in thepositive direction when a potential lower than a source potential isapplied to the back gate. In contrast, the threshold voltage is shiftedin the negative direction when a potential higher than a sourcepotential is applied to the back gate. Accordingly, in the case wherethe on/off state of each transistor is controlled with a predeterminedgate voltage, the off-state current can be reduced when a potentiallower than a source potential is supplied to a back gate, and theon-state current can be increased when a potential higher than a sourcepotential is supplied to the back gate.

It is preferred that the floating nodes Fn1 and Fn2 in thelight-receiving circuit 21 be highly capable of retaining a potential;thus, OS transistors with a small off-state current are preferably usedas the transistors 41 to 43 as described above. When a potential lowerthan a source potential is supplied to the back gates of the transistors41 to 43, the off-state current can be further reduced; thus, thepotential retention capability of the floating nodes Fn1 and Fn2 can beenhanced.

As described above, for example, transistors with a large on-statecurrent are preferably used as the transistors 44 a and 45 a included inthe amplifier circuit 22 a as illustrated in FIG. 18C. The on-statecurrent can be increased when a potential higher than a source potentialis applied to back gates of the transistors 44 a and 45 a. FIG. 18Cillustrates an example in which a terminal VBG 73 is connected to theback gates of the transistors included in the light-receiving circuit 21and a terminal VBG 73 a is connected to the back gates of thetransistors included in the amplifier circuit 22 a; however, the backgates of the transistors can be supplied with different potentials.Thus, an increase in on-state current can improve responsecharacteristics of the amplifier circuit 22 a, and the amplifier circuit22 a can operate at high frequency.

In order to increase the light sensitivity of an imaging device, theamount of current flowing through photodiodes can be controlled bychanging voltage applied between the photodiodes; thus, an appropriatelight sensitivity can be set in accordance with detection data of anenvironmental sensor (e.g., an illumination sensor, a temperaturesensor, or a humidity sensor) that senses and supervises the useenvironment.

In addition to power supply potentials, a plurality of potentials suchas a signal potential and a potential applied to the back gate are usedinside an imaging device. Supply of a plurality of potentials from theoutside of an imaging device increases the number of terminals; thus, animaging device preferably has a power supply circuit generating aplurality of potentials inside the imaging device.

As illustrated in FIG. 19A, the transistors 41 and 42 included in thelight-receiving circuit 21 may be connected to each other. One of asource and a drain of the transistor 41, one of a source and a drain ofthe transistor 42, and a gate of the transistor 44 a are electricallyconnected to each other to form the floating node Fn1.

As illustrated in FIG. 19B, the terminal VRS 72 in FIG. 19A may bedirectly connected to a gate of the transistor 45 a.

FIG. 20A is a block diagram illustrating an example of the A/D convertercircuit 26. The A/D converter circuit 26 can include a comparator 26 a,a counter circuit 26 b, and the like and output digital data of two bitsor more to a wiring 93 (OUT3).

The comparator 26 a compares a signal potential that is input from aterminal 37 to a terminal 38 and a reference potential (VREF) that isswept up or down. Then, the counter circuit 26 b operates in accordancewith the output of the comparator 26 a and outputs a digital signal tothe wiring 93 (OUT3).

To achieve high-speed operation and power saving, the A/D convertercircuit 26 is preferably constituted by Si transistors capable offorming a CMOS circuit.

The imaging element 10 and the A/D converter circuit 26 can be connectedin such a manner, for example, that the terminals 37 and the terminals38 are connected through wires by a wire bonding method or the like asillustrated in FIG. 25B.

In Embodiment 3, one embodiment of the present invention has beendescribed. Other embodiments of the present invention will be describedin Embodiments 1, 2, and 4 to 8. Note that one embodiment of the presentinvention is not limited to these embodiments. In other words, variousembodiments of the invention are described in this embodiment andEmbodiments 1, 2, and 4 to 8, and one embodiment of the presentinvention is not limited to a particular embodiment. Although an examplein which one embodiment of the present invention is applied to animaging device is described, one embodiment of the present invention isnot limited thereto. Depending on circumstances or conditions, oneembodiment of the present invention is not necessarily applied to animaging device. One embodiment of the present invention may be appliedto a semiconductor device with another function, for example. Althoughan example in which a channel formation region, a source region, a drainregion, and the like of a transistor contain an oxide semiconductor isdescribed as one embodiment of the present invention, one embodiment ofthe present invention is not limited thereto. Depending on circumstancesor conditions, various transistors or a channel formation region, asource region, a drain region, and the like of a transistor in oneembodiment of the present invention may contain a variety ofsemiconductors. Depending on circumstances or conditions, varioustransistors or a channel formation region, a source region, a drainregion, and the like of a transistor in one embodiment of the presentinvention may contain at least one of silicon, germanium, silicongermanium, silicon carbide, gallium arsenide, aluminum gallium arsenide,indium phosphide, gallium nitride, and an organic semiconductor, forexample, or alternatively do not necessarily contain an oxidesemiconductor. Although an example in which a global shutter system isemployed is described as one embodiment of the present invention, oneembodiment of the present invention is not limited thereto. Depending oncircumstances or conditions, another system such as a rolling shuttersystem may be employed in one embodiment of the present invention, oralternatively a global shutter system is not necessarily employed.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 4

In this embodiment, an OS transistor that can be used in one embodimentof the present invention will be described with reference to drawings.In the drawings in this embodiment, some components are enlarged,reduced in size, or omitted for easy understanding.

FIGS. 21A to 21C are a top view and cross-sectional views of atransistor 101 of one embodiment of the present invention. FIG. 21A isthe top view. FIG. 21B shows a cross section along the dashed-dottedline X1-X2 in FIG. 21A. FIG. 21C shows a cross section along thedashed-dotted line Y1-Y2 in FIG. 21A.

In the drawings explained in this embodiment, the direction of thedashed-dotted line X1-X2 is referred to as channel length direction, andthe direction of the dashed-dotted line Y1-Y2 is referred to as channelwidth direction.

The transistor 101 includes an insulating layer 120 in contact with thesubstrate 115, the conductive layer 173 in contact with the insulatinglayer 120, the oxide semiconductor layer 130 in contact with theinsulating layer 120, the conductive layers 140 and 150 electricallyconnected to the oxide semiconductor layer 130, the insulating layer 160in contact with the oxide semiconductor layer 130 and the conductivelayers 140 and 150, and the conductive layer 170 in contact with theinsulating layer 160.

Over the transistor 101, an insulating layer 180 in contact with theoxide semiconductor layer 130, the conductive layers 140 and 150, theinsulating layer 160, and the conductive layer 170 may be provided asnecessary.

The oxide semiconductor layer 130 can have a three-layer structure ofoxide semiconductor layers 130 a, 130 b, and 130 c, for example.

The conductive layers 140 and 150 can function as a source electrodelayer and a drain electrode layer. The insulating layer 160 and theconductive layer 170 can function as a gate insulating film and a gateelectrode layer, respectively.

Using the conductive layer 173 as a second gate electrode layer (backgate) enables the increase in on-state current and control of thethreshold voltage. Note that the conductive layer 173 can also serve asa light-blocking layer.

In order to increase the on-state current, for example, the conductivelayers 170 and 173 are made to have the same potential, and thetransistor is driven as a double-gate transistor. Furthermore, in orderto control the threshold voltage, a fixed potential that is differentfrom the potential of the conductive layer 170 is supplied to theconductive layer 173.

In the oxide semiconductor layer 130, a region in contact with theconductive layer 140 and a region in contact with the conductive layer150 can function as a source region and a drain region.

Since the oxide semiconductor layer 130 is in contact with theconductive layers 140 and 150, an oxygen vacancy is generated in theoxide semiconductor layer 130, and the regions become n-typelow-resistance regions owing to interaction between the oxygen vacancyand hydrogen that remains in the oxide semiconductor layer 130 ordiffuses into the oxide 30 semiconductor layer 130 from the outside.

Note that functions of a “source” and a “drain” of a transistor aresometimes replaced with each other when a transistor of oppositepolarity is used or when the direction of current flow is changed incircuit operation, for example. Therefore, the terms “source” and“drain” can be interchanged with each other in this specification. Inaddition, the term “electrode layer” can be replaced with the term“wiring”.

The conductive layers 140 and 150 are in contact with a top surface ofthe oxide semiconductor layer 130 and are not in contact with sidesurfaces of the oxide semiconductor layer 130. This structurefacilitates compensation for oxygen vacancies in the oxide semiconductorlayer 130 with oxygen included in the insulating layer 120.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 22A to 22C. FIG. 22A is a top view of atransistor 102. FIG. 22B shows a cross section along the dashed-dottedline X1-X2 in FIG. 22A. FIG. 22C shows a cross section along thedashed-dotted line Y1-Y2 in FIG. 22A.

The transistor 102 has the same structure as the transistor 101 exceptthat the conductive layers 140 and 150 are in contact with theinsulating layer 120, and that the conductive layers 140 and 150 are incontact with side surfaces of the oxide semiconductor layer 130.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 23A to 23C. FIG. 23A is a top view of atransistor 103. FIG. 23B shows a cross section along the dashed-dottedline X1-X2 in FIG. 23A. FIG. 23C shows a cross section along thedashed-dotted line Y1-Y2 in FIG. 23A.

The transistor 103 has the same structure as the transistor 101 exceptthat the oxide semiconductor layers 130 a and 130 b and the conductivelayers 140 and 150 are covered with the oxide semiconductor layer 130 cand the insulating layer 160.

When the oxide semiconductor layer 130 c covers the oxide semiconductorlayers 130 a and 130 b, the effect of compensating for oxygen to theoxide semiconductor layers 130 a and 130 b and the insulating layer 120can be enhanced. Moreover, oxidation of the conductive layers 140 and150 by the insulating layer 180 can be suppressed with the oxidesemiconductor layer 130 c placed therebetween.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 24A to 24C. FIG. 24A is a top view of atransistor 104. FIG. 24B shows a cross section along the dashed-dottedline X1-X2 in FIG. 24A. FIG. 24C shows a cross section along thedashed-dotted line Y1-Y2 in FIG. 24A.

The transistor 104 has the same structure as the transistor 101 exceptthat the oxide semiconductor layers 130 a and 130 b and the conductivelayers 140 and 150 are covered with the oxide semiconductor layer 130 c,and that the insulating layer 170 is covered with an insulating layer210.

The insulating layer 210 can be formed using a material with a blockingproperty against oxygen, for example, metal oxide such as aluminumoxide. Oxidation of the conductive layer 170 by the insulating layer 180can be suppressed with the insulating layer 210 placed therebetween.

The transistors 101 to 104 each have a top-gate structure including aregion where the conductive layer 170 overlaps the conductive layers 140and 150. To reduce parasitic capacitance, the width of the region in thechannel length direction is preferably greater than or equal to 3 nm andless than 300 nm. Since an offset region is not formed in the oxidesemiconductor layer 130 in this structure, a transistor with a largeon-state current can be easily formed.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 25A to 25C. FIG. 25A is a top view of atransistor 105. FIG. 25B shows a cross section along the dashed-dottedline X1-X2 in FIG. 25A. FIG. 25C shows a cross section along thedashed-dotted line Y1-Y2 in FIG. 25A.

The transistor 105 includes the insulating layer 120 in contact with thesubstrate 115, the conductive layer 173 in contact with the insulatinglayer 120, the oxide semiconductor layer 130 in contact with theinsulating layer 120, the insulating layer 160 in contact with the oxidesemiconductor layer 130, and the conductive layer 170 in contact withthe insulating layer 160.

In the insulating layer 180 serving as an interlayer insulating film, aconductor 200 in 30 contact with a region 231 of the oxide semiconductorlayer 130 and a conductor 201 in contact with a region 232 of the oxidesemiconductor layer 130 are provided. The conductors 200 and 201 canfunction as part of the source electrode layer and part of the drainelectrode layer.

An impurity for forming an oxygen vacancy to increase conductivity ispreferably added to the regions 231 and 232 in the transistor 105. As animpurity for forming an oxygen vacancy in an oxide semiconductor layer,one or more of the following can be used, for example: phosphorus,arsenic, antimony, boron, aluminum, silicon, nitrogen, helium, neon,argon, krypton, xenon, indium, fluorine, chlorine, titanium, zinc, andcarbon. As a method for adding the impurity, plasma treatment, ionimplantation, ion doping, plasma immersion ion implantation, or the likecan be used.

When the above element is added as an impurity element to the oxidesemiconductor layer, a bond between a metal element and oxygen in theoxide semiconductor layer is cut, so that an oxygen vacancy is formed.Interaction between an oxygen vacancy in the oxide semiconductor layerand hydrogen that remains in the oxide semiconductor layer or is addedto the oxide semiconductor layer later can increase the conductivity ofthe oxide semiconductor layer.

When hydrogen is added to an oxide semiconductor in which an oxygenvacancy is formed by addition of an impurity element, hydrogen enters anoxygen vacant site and forms a donor level in the vicinity of theconduction band. Consequently, an oxide conductor can be formed. Here,an oxide conductor refers to an oxide semiconductor that has become aconductor.

The transistor 105 has a self-aligned structure that does not include aregion where the conductive layer 170 overlaps the conductive layers 140and 150. A transistor with a self-aligned structure, which has extremelylow parasitic capacitance between a gate electrode layer and source anddrain electrode layers, is suitable for applications that requirehigh-speed operation.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 26A to 26C. FIG. 26A is a top view of atransistor 106. FIG. 26B shows a cross section along the dashed-dottedline X1-X2 in FIG. 26A. FIG. 26C shows a cross section along thedashed-dotted line Y1-Y2 in FIG. 26A.

The transistor 106 includes the substrate 115, the insulating layer 120over the substrate 115, the conductive layer 173 in contact with theinsulating layer 120, the oxide semiconductor layer 130 (the oxidesemiconductor layers 130 a, 130 b, and 130 c) over the insulating layer120, the conductive layers 140 and 150 that are in contact with theoxide semiconductor layer 130 and are apart from each other, theinsulating layer 160 in contact with the oxide semiconductor layer 130c, and the conductive layer 170 in contact with the insulating layer160.

Note that the oxide semiconductor layer 130 c, the insulating layer 160,and the conductive layer 170 are provided in an opening that is providedin the insulating layer 180 over the transistor 106 and reaches theoxide semiconductor layers 130 a and 130 b and the insulating layer 120.

The transistor in one embodiment of the present invention may have astructure illustrated in FIGS. 27A to 27C. FIG. 27A is a top view of atransistor 107. FIG. 27B shows a cross section along the dashed-dottedline X1-X2 in FIG. 27A. FIG. 27C shows a cross section along thedashed-dotted line Y1-Y2 in FIG. 27A.

The transistor 107 has the same structure as the transistor 106 exceptthat the oxide semiconductor layers 130 a and 130 b and the conductivelayers 140 and 150 are covered with the oxide semiconductor layer 130 cand an oxide semiconductor layer 130 d. The oxide semiconductor layer130 d can be formed using the same material as the oxide semiconductorlayer 130 c.

When the oxide semiconductor layers 130 c and 130 d cover the oxidesemiconductor layers 130 a and 130 b, the effect of compensating foroxygen to the oxide semiconductor layers 130 a and 130 b and theinsulating layer 120 can be enhanced. Moreover, oxidation of theconductive layers 140 and 150 by the insulating layer 180 can besuppressed with the oxide semiconductor layer 130 d placed therebetween.

The transistors 106 and 107 each have a smaller region in which aconductor serving as a source or a drain overlaps with a conductorserving as a gate electrode; thus, parasitic capacitance in thetransistors 106 and 107 can be reduced. Consequently, the transistors106 and 107 are suitable for components of a circuit that requireshigh-speed operation.

In the transistor of one embodiment of the present invention, the oxidesemiconductor layer 130 may be a single layer as illustrated in FIG.28A, or may be formed of two layers as illustrated in FIG. 28B.

The transistor of one embodiment of the present invention may beconfigured not to include the conductive layer 173 as illustrated inFIG. 28C.

To electrically connect the conductive layer 170 to the conductive layer173 in the transistor of one embodiment of the present invention, forexample, an opening is formed in the insulating layer 120, the oxidesemiconductor layer 130 c, and the insulating layer 160 to reach theconductive layer 173, and the conductive layer 170 is formed to coverthe opening as illustrated in FIG. 28D.

The transistor of one embodiment of the present invention may beprovided with an insulating layer 145 and an insulating layer 155 thatare in contact with the conductive layer 140 and the conductive layer150, respectively, as illustrated in FIG. 28E. The insulating layers 145and 155 can prevent oxidation of the conductive layers 140 and 150.

The insulating layers 145 and 155 can be formed using a material with ablocking property against oxygen, for example, metal oxide such asaluminum oxide.

In the transistor of one embodiment of the present invention, theconductive layer 170 may be a stack including a conductive layer 171 anda conductive layer 172, as illustrated in FIG. 28F.

In the transistor of one embodiment of the present invention, in whichthe conductive layers 140 and 150 are provided over the oxidesemiconductor layer 130, the width (W_(SD)) of the conductive layers 140and 150 may be smaller than the width (W_(OS)) of the oxidesemiconductor layer 130 as shown in top views of FIGS. 28G and 28H (thatonly show the oxide semiconductor layer 130 and the conductive layers140 and 150). When W_(OS)≥W_(SD) is satisfied (W_(SD) is smaller than orequal to W_(OS)), a gate electric field is easily applied to the entirechannel formation region, so that electrical characteristics of thetransistor can be improved.

FIGS. 28A to 28F illustrate variation examples of the transistor 101;these examples can also be applied to the other transistors described inthis embodiment.

In the transistor with any of the above structures in one embodiment ofthe present invention, the conductive layer 170 (and the conductivelayer 173) serving as a gate electrode layer electrically surrounds theoxide semiconductor layer 130 in the channel width direction with theinsulating layer therebetween. Such a structure can increase theon-state current and is referred to as a surrounded channel (s-channel)structure.

In the transistor including the oxide semiconductor layers 130 a and 130b and the transistor including the oxide semiconductor layers 130 a, 130b, and 130 c, selecting appropriate materials for the two or threelayers forming the oxide semiconductor layer 130 allows current to flowin the oxide semiconductor layer 130 b. Since current flows through theoxide semiconductor layer 130 b, the current is hardly influenced byinterface scattering, leading to a large on-state current.

A semiconductor device including the transistor with any of the abovestructures can have favorable electrical characteristics.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 5

In this embodiment, components of the transistors shown in Embodiment 3will be described in detail.

As the substrate 115, a glass substrate, a quartz substrate, asemiconductor substrate, a ceramic substrate, a metal substrate with aninsulated surface, or the like can be used. Other examples are a siliconsubstrate provided with a transistor and/or a photodiode; and a siliconsubstrate over which an insulating layer, a wiring, a conductorfunctioning as a contact plug, and the like are provided together with atransistor and/or a photodiode. When a p-channel transistor is formed ona silicon substrate, a silicon substrate with n⁻-type conductivity ispreferably used. Alternatively, an SOI substrate including an n⁻-type ori-type silicon layer may be used. Moreover, when a p-channel transistoris formed on a silicon substrate, a surface of the silicon substratewhere the transistor is formed preferably has a (110) plane orientation,in which case the mobility can be increased.

The insulating layer 120 can have a function of supplying oxygen to theoxide semiconductor layer 130 as well as a function of preventingdiffusion of impurities from a component of the substrate 115. For thisreason, the insulating layer 120 preferably contains oxygen and morepreferably has an oxygen content higher than that in the stoichiometriccomposition. The insulating layer 120 is a film in which the amount ofreleased oxygen when converted into oxygen atoms is preferably greaterthan or equal to 1.0×10¹⁹ atoms/cm³ in TDS analysis. In the TDSanalysis, heat treatment is performed such that the film surfacetemperature ranges from 100° C. to 700° C., preferably from 100° C. to500° C. When the substrate 115 is provided with another device, theinsulating layer 120 also functions as an interlayer insulating film. Inthat case, the insulating layer 120 is preferably subjected toplanarization treatment such as CMP treatment so as to have a flatsurface.

As the conductive layer 173 functioning as a back gate electrode layer,a conductive film formed using Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru,Ag, Mn, Nd, Sc, Ta, or W can be used, for example. It is also possibleto use an alloy or a conductive nitride of any of the above materials,or a stack including a plurality of materials selected from thesematerials, alloys of these materials, and conductive nitrides of thesematerials.

For example, the insulating layer 120 can be formed using an oxideinsulating film containing aluminum oxide, magnesium oxide, siliconoxide, silicon oxynitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,tantalum oxide, or the like; a nitride insulating film containingsilicon nitride, silicon nitride oxide, aluminum nitride, aluminumnitride oxide, or the like; or a mixed material of any of these. Theinsulating layer 120 may be a stack of any of the above materials.

The oxide semiconductor layer 130 can have a three-layer structure inwhich the oxide semiconductor layers 130 a, 130 b, and 130 c are stackedin this order from the insulating layer 120 side.

Note that when the oxide semiconductor layer 130 is a single layer, alayer corresponding to the oxide semiconductor layer 130 b described inthis embodiment is used.

In the case of employing a two-layer structure, the oxide semiconductorlayer 130 can be a stack in which a layer corresponding to the oxidesemiconductor layer 130 a and a layer corresponding to the oxidesemiconductor layer 130 b are stacked in this order from the insulatinglayer 120 side. In this structure, the positions of the oxidesemiconductor layers 130 a and 130 b can be interchanged.

For the oxide semiconductor layer 130 b, an oxide semiconductor whoseelectron affinity (energy difference between the vacuum level and theconduction band minimum) is higher than those of the oxide semiconductorlayers 130 a and 130 c is used, for example.

In such a structure, when a voltage is applied to the conductive layer170, a channel is formed in the oxide semiconductor layer 130 b whoseconduction band minimum is the lowest in the oxide semiconductor layer130. Therefore, the oxide semiconductor layer 130 b can be regarded ashaving a region serving as a semiconductor, while the oxidesemiconductor layer 130 a and the oxide semiconductor layer 130 c can beregarded as having a region serving as an insulator or a semi-insulator.

An oxide semiconductor that can be used for each of the oxidesemiconductor layers 130 a, 130 b, and 130 c preferably contains atleast one of In and Zn, or both In and Zn. In order to reduce variationsin electrical characteristics of the transistor including the oxidesemiconductor, the oxide semiconductor preferably contains a stabilizersuch as Al, Ga, Y, or Sn in addition to In and/or Zn.

For the oxide semiconductor layers 130 a and 130 c, In—Ga—Zn oxide withan atomic ratio of In to Ga and Zn (In:Ga:Zn) of 1:3:2, 1:3:3, 1:3:4,1:3:6, 1:4:5, 1:6:4, or 1:9:6 or close to these ratios can be used, forexample. For the oxide semiconductor layer 130 b, In—Ga—Zn oxide with anatomic ratio In:Ga:Zn of 1:1:1, 2:1:3, 5:5:6, 3:1:2, 3:1:4, 5:1:6, or4:2:3 or close to these ratios can be used, for example.

The oxide semiconductor layers 130 a, 130 b, and 130 c may includecrystal parts. For example, when crystals with c-axis alignment areused, the transistor can have stable electrical characteristics.Moreover, crystals with c-axis alignment are resistant to bending; thus,using such crystals can improve the reliability of a semiconductordevice using a flexible substrate.

The conductive layer 140 functioning as a source electrode and theconductive layer 150 functioning as a drain electrode can be formed witha single layer or a stacked layer using a material selected from Al, Cr,Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, and Sc and alloys or conductive nitridesof any of these metal materials, for example. Using tantalum nitride,which is a conductive nitride, can prevent oxidation of the conductivelayers 140 and 150. It is also possible to use a stack of any of theabove materials and Cu or an alloy such as Cu—Mn, which has lowresistance.

The above materials are capable of extracting oxygen from an oxidesemiconductor layer. Thus, in a region of the oxide semiconductor layerthat is in contact with any of the above materials, oxygen is releasedfrom the oxide semiconductor layer and an oxygen vacancy is formed.Hydrogen slightly contained in the oxide semiconductor layer and theoxygen vacancy are bonded to each other, so that the region is markedlychanged to an n-type region. Accordingly, the n-type region can serve asa source or a drain of the transistor.

The insulating layer 160 functioning as a gate insulating film can be aninsulating film containing one or more of aluminum oxide, magnesiumoxide, silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating layer 160 may be a stack including any of the abovematerials.

As the insulating layers 120 and 160 in contact with the oxidesemiconductor layer 130, a film that releases less nitrogen oxide ispreferably used. When the oxide semiconductor is in contact with aninsulating layer that releases a large amount of nitrogen oxide, thedensity of states caused by nitrogen oxide becomes high in some cases.

By using the above insulating film as the insulating layers 120 and 160,a shift in the threshold voltage of the transistor can be reduced, whichleads to reduced fluctuations in the electrical characteristics of thetransistor.

As the conductive layer 170 functioning as a gate electrode layer, aconductive film of Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Mn, Nd,Sc, Ta, or W can be used, for example. An alloy or a conductive nitrideof any of these materials can also be used. A stack containing aplurality of materials selected from the above materials, alloys ofthese materials, and conductive nitrides of these materials can also beused. As a typical example, tungsten, a stack of tungsten and titaniumnitride, or a stack of tungsten and tantalum nitride can be used.Alternatively, Cu or an alloy such as Cu—Mn, which has low resistance,or a stack of any of the above materials and Cu or an alloy such asCu—Mn may be used. For example, titanium nitride can be used for theconductive layer 171 and tungsten can be used for the conductive layer172 to form the conductive layer 170.

As the conductive layer 170, an oxide conductive layer of In—Ga—Znoxide, zinc oxide, indium oxide, tin oxide, indium tin oxide, or thelike may be used. When the oxide conductive layer is provided in contactwith the insulating layer 160, oxygen can be supplied from the oxideconductive layer to the oxide semiconductor layer 130.

The insulating layer 180 can be an insulating film containing one ormore of magnesium oxide, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,and tantalum oxide. The insulating layer 180 may be a stack of any ofthe above materials.

Here, like the insulating layer 120, the insulating layer 180 preferablycontains oxygen more than that in the stoichiometric composition. Oxygenreleased from the insulating layer 180 can be diffused into the channelformation region in the oxide semiconductor layer 130 through theinsulating layer 160, so that oxygen vacancies formed in the channelformation region can be filled with oxygen. In this manner, stableelectrical characteristics of the transistor can be achieved.

A film having an effect of blocking impurities is preferably providedover the transistor or the insulating layer 180. The blocking film canbe a silicon nitride film, an aluminum nitride film, an aluminum oxidefilm, or the like.

A nitride insulating film has a function of blocking moisture and thelike and can improve the reliability of the transistor. An aluminumoxide film has a high blocking effect of preventing penetration of bothoxygen and impurities such as hydrogen and moisture. Accordingly, duringand after the manufacturing process of the transistor, the aluminumoxide film can suitably function as a protective film that has effectsof preventing entry of impurities such as hydrogen and moisture into theoxide semiconductor layer 130, preventing release of oxygen from theoxide semiconductor layer, and preventing unnecessary release of oxygenfrom the insulating layer 120.

High integration of a semiconductor device requires transistorminiaturization. However, miniaturization of a transistor tends to causedeterioration of electrical characteristics of the transistor. Forexample, a decrease in channel width causes a reduction in on-statecurrent.

In the transistor of one embodiment of the present invention, the oxidesemiconductor layer 130 c can cover the oxide semiconductor layer 130 bin which the channel is formed. In this structure, the channel formationlayer is not in contact with the gate insulating film; thus, scatteringof carriers formed at the interface between the channel formation layerand the gate insulating film can be reduced, and the on-state current ofthe transistor can be increased.

In the transistor of one embodiment of the present invention, asdescribed above, the gate electrode layer (the conductive layer 170) isformed to electrically surround the oxide semiconductor layer 130 in thechannel width direction. Accordingly, a gate electric field is appliedto the oxide semiconductor layer 130 in a direction perpendicular to itsside surface in addition to a direction perpendicular to its topsurface. In other words, a gate electric field is applied to the entirechannel formation layer and effective channel width is increased,leading to a further increase in the on-state current.

Although the variety of films such as the metal films, the semiconductorfilms, and the inorganic insulating films described in this embodimentcan be formed typically by a sputtering method or a plasma-enhanced CVDmethod, such films may be formed by another method such as a thermal CVDmethod. Examples of a thermal CVD method include a metal organicchemical vapor deposition (MOCVD) method and an atomic layer deposition(ALD) method.

Since plasma is not used for deposition, a thermal CVD method has anadvantage that no defect due to plasma damage is generated.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to a chamber at a time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is caused in the vicinity of thesubstrate or over the substrate.

Deposition by an ALD method is performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are introduced into the chamber andreacted, and then the sequence of gas introduction is repeated. An inertgas (e.g., argon or nitrogen) may be introduced as a carrier gastogether with the source gases. For example, two or more kinds of sourcegases may be sequentially supplied to the chamber. In that case, afterreaction of a first source gas, an inert gas is introduced, and then asecond source gas is introduced so that the source gases are not mixed.Alternatively, the first source gas may be exhausted by vacuumevacuation instead of introduction of an inert gas, and then the secondsource gas may be introduced. The first source gas is adsorbed on thesurface of the substrate and reacted to form a first layer, and then,the second source gas introduced is adsorbed on the first layer andreacted. As a result, a second layer is stacked over the first layer, sothat a thin film is formed. The sequence of the gas introduction iscontrolled and repeated more than once until a desired thickness isobtained, whereby a thin film with excellent step coverage can beformed. The thickness of the thin film can be adjusted by the number ofrepetitions of the sequence of gas introduction; therefore, ALD makes itpossible to accurately adjust a film thickness and thus is suitable formanufacturing a minute FET.

A facing-target-type sputtering apparatus can be used to form an oxidesemiconductor layer. Deposition using a facing-target-type sputteringapparatus can be referred to as vapor deposition sputtering (VDSP).

When an oxide semiconductor layer is deposited using afacing-target-type sputtering apparatus, plasma damage to the oxidesemiconductor layer at the time of deposition can be reduced. Thus,oxygen vacancies in the layer can be reduced. In addition, the use ofthe facing-target-type sputtering apparatus enables low-pressuredeposition. Accordingly, the concentration of impurities (e.g.,hydrogen, a rare gas such as argon, and water) in a deposited oxidesemiconductor layer can be lowered.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 6

In this specification and the like, a metal oxide means an oxide ofmetal in a broad sense. Metal oxides are classified into an oxideinsulator, an oxide conductor (including a transparent oxide conductor),an oxide semiconductor (also simply referred to as an OS), and the like.For example, a metal oxide used in a semiconductor layer of a transistoris called an oxide semiconductor in some cases. That is to say, a metaloxide that has at least one of an amplifying function, a rectifyingfunction, and a switching function can be called a metal oxidesemiconductor, or OS for short. In addition, an OS transistor is atransistor including a metal oxide or an oxide semiconductor.

In this specification and the like, a metal oxide including nitrogen isalso called a metal oxide in some cases. Moreover, a metal oxideincluding nitrogen may be called a metal oxynitride.

In this specification and the like, “c-axis aligned crystal (CAAC)” or“cloud-aligned composite (CAC)” may be stated in some cases. CAAC refersto an example of a crystal structure, and CAC refers to an example of afunction or a material composition.

In this specification and the like, a CAC-OS or a CAC metal oxide has aconducting function in part of the material and has an insulatingfunction in another part of the material; as a whole, the CAC-OS or theCAC metal oxide has a function of a semiconductor. In the case where theCAC-OS or the CAC metal oxide is used in a semiconductor layer of atransistor, the conducting function is to allow electrons (or holes)serving as carriers to flow, and the insulating function is to not allowelectrons serving as carriers to flow. By the complementary action ofthe conducting function and the insulating function, the CAC-OS or theCAC metal oxide can have a switching function (on/off function). In theCAC-OS or the CAC metal oxide, separation of the functions can maximizeeach function.

The CAC-OS or the CAC metal oxide includes components having differentbandgaps.

For example, the CAC-OS or the CAC metal oxide includes a componenthaving a wide gap due to the insulating region and a component having anarrow gap due to the conductive region. In the case of such acomposition, carriers mainly flow in the component having a narrow gap.The component having a narrow gap complements the component having awide gap, and carriers also flow in the component having a wide gap inconjunction with the component having a narrow gap. Therefore, in thecase where the above-described CAC-OS or the CAC metal oxide is used ina channel region of a transistor, high current drive capability in theon state of the transistor, that is, a large on-state current and a highfield-effect mobility, can be obtained.

In other words, a CAC-OS or a CAC metal oxide can be called a matrixcomposite or a metal matrix composite.

<Composition of CAC-OS>

Described below is the composition of a CAC-OS, which can be used forthe transistor disclosed in one embodiment of the present invention.

The CAC-OS has, for example, a composition in which elements included inan oxide semiconductor are unevenly distributed. Materials includingunevenly distributed elements each have a size greater than or equal to0.5 nm and less than or equal to 10 nm, preferably greater than or equalto 1 nm and less than or equal to 2 nm, or a similar size. Note that inthe following description of an oxide semiconductor, a state in whichone or more metal elements are unevenly distributed and regionsincluding the metal element(s) are mixed is referred to as a mosaicpattern or a patch-like pattern. The region has a size greater than orequal to 0.5 nm and less than or equal to 10 nm, preferably greater thanor equal to 1 nm and less than or equal to 2 nm, or a similar size.

Note that an oxide semiconductor preferably contains at least indium. Inparticular, indium and zinc are preferably contained. In addition, oneor more of aluminum, gallium, yttrium, copper, vanadium, beryllium,boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like may be contained.

For example, of the CAC-OS, an In—Ga—Zn oxide with the CAC composition(such an In—Ga—Zn oxide may be particularly referred to as CAC-IGZO) hasa composition in which materials are separated into indium oxide(InO_(X1), where X1 is a real number greater than 0) or indium zincoxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), and gallium oxide (GaO_(X3), where X3 is a real numbergreater than 0) or gallium zinc oxide (Ga_(X4)Zn_(Y4)O_(Z4), where X4,Y4, and Z4 are real numbers greater than 0), and a mosaic pattern isformed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) forming the mosaicpattern is evenly distributed in the film. This composition is alsoreferred to as a cloud-like composition.

That is, the CAC-OS is a composite oxide semiconductor with acomposition in which a region including GaO_(X3) as a Main Component anda Region Including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main componentare mixed. Note that in this specification, for example, when the atomicratio of In to an element M in a first region is greater than the atomicratio of In to an element M in a second region, the first region hashigher In concentration than the second region.

Note that a compound including In, Ga, Zn, and O is also known as IGZO.Typical examples of IGZO include a crystalline compound represented byInGaO₃(ZnO)_(m1) (m1 is a natural number) and a crystalline compoundrepresented by In_((1+x0))Ga_((1−x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is agiven number).

The above crystalline compounds have a single crystal structure, apolycrystalline structure, or a CAAC structure. Note that the CAACstructure is a crystal structure in which a plurality of IGZOnanocrystals have c-axis alignment and are connected in the a-b planedirection without alignment.

On the other hand, the CAC-OS relates to the material composition of anoxide semiconductor. In a material composition of a CAC-OS including In,Ga, Zn, and O, nanoparticle regions including Ga as a main component areobserved in part of the CAC-OS and nanoparticle regions including In asa main component are observed in part thereof. These nanoparticleregions are randomly dispersed to form a mosaic pattern. Therefore, thecrystal structure is a secondary element for the CAC-OS.

Note that in the CAC-OS, a stacked structure including two or more filmswith different atomic ratios is not included. For example, a two-layerstructure of a film including In as a main component and a filmincluding Ga as a main component is not included.

A boundary between the region including GaO_(X3) as a main component andthe region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is not clearly observed in some cases.

In the case where one or more of aluminum, yttrium, copper, vanadium,beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like are contained instead of gallium in a CAC-OS,nanoparticle regions including the selected metal element(s) as a maincomponent(s) are observed in part of the CAC-OS and nanoparticle regionsincluding In as a main component are observed in part thereof, and thesenanoparticle regions are randomly dispersed to form a mosaic pattern inthe CAC-OS.

The CAC-OS can be formed by a sputtering method under conditions where asubstrate is not heated intentionally, for example. In the case offorming the CAC-OS by a sputtering method, one or more selected from aninert gas (typically, argon), an oxygen gas, and a nitrogen gas may beused as a deposition gas. The flow rate of an oxygen gas to the totalflow rate of the deposition gas at the time of deposition is preferablyas low as possible, and for example, the flow rate of an oxygen gas ispreferably higher than or equal to 0% and lower than 30%, morepreferably higher than or equal to 0% and lower than or equal to 10%.

The CAC-OS is characterized in that no clear peak is observed inmeasurement using θ/2θ scan by an out-of-plane method, which is an X-raydiffraction (XRD) measurement method. That is, X-ray diffraction showsno alignment in the a-b plane direction and the c-axis direction in ameasured region.

In an electron diffraction pattern of the CAC-OS which is obtained byirradiation with an electron beam with a probe diameter of 1 nm (alsoreferred to as a nanometer-sized electron beam), a ring-like region withhigh luminance and a plurality of bright spots in the ring-like regionare observed. Therefore, the electron diffraction pattern indicates thatthe crystal structure of the CAC-OS includes a nanocrystal (nc)structure with no alignment in plan-view and cross-sectional directions.

For example, an energy dispersive X-ray spectroscopy (EDX) mapping imageconfirms that an In—Ga—Zn oxide with the CAC composition has a structurein which the region including GaO_(X3) as a main component and theregion including In_(X2)Zn_(Y2)O_(X2) or InO_(X1) as a main componentare unevenly distributed and mixed.

The CAC-OS has a structure different from that of an IGZO compound inwhich metal elements are evenly distributed, and has characteristicsdifferent from those of the IGZO compound. That is, in the CAC-OS,regions including GaO_(X3) or the like as a main component and regionsincluding In_(X2)Zn_(Y2)O_(X2) or InO_(X1) as a main component areseparated to form a mosaic pattern.

The conductivity of the region including In_(X2)Zn_(Y2)O_(X2) orInO_(X1) as a main component is higher than that of the region includingGaO_(X3) or the like as a main component. In other words, when carriersflow through the region including In_(X2)Zn_(Y2)O_(X2) or InO_(X1) as amain component, the conductivity of an oxide semiconductor is exhibited.Accordingly, when the regions including In_(X2)Zn_(Y2)O_(X2) or InO_(X1)as a main component are distributed in the oxide semiconductor like acloud, a high field-effect mobility (μ) can be achieved.

In contrast, the insulating property of the region including GaO_(X3) orthe like as a main component is higher than that of the region includingIn_(X2)Zn_(Y2)O_(X2) or InO_(X1) as a main component. In other words,when the regions including GaO_(X3) or the like as a main component aredistributed in the oxide semiconductor, leakage current can besuppressed and favorable switching operation can be achieved.

Accordingly, when a CAC-OS is used for a semiconductor element, theinsulating property derived from GaO_(X3) or the like and theconductivity derived from In_(X2)Zn_(Y2)O_(X2) or InO_(X1) complementeach other, whereby a large on-state current (I_(on)) and a highfield-effect mobility (μ) can be achieved.

A semiconductor element including a CAC-OS has a high reliability. Thus,the CAC-OS is suitably used in a variety of semiconductor devicestypified by a display.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 7

In this embodiment, examples of a package and a camera module eachincluding an image sensor chip are described. For the image sensor chip,the structure of an imaging device of one embodiment of the presentinvention can be used.

FIG. 29A is an external perspective view showing the top surface side ofa package including an image sensor chip. The package includes a packagesubstrate 810 to which an image sensor chip 850 is fixed, a cover glass820, an adhesive 830 for bonding the package substrate 810 and the coverglass 820 to each other, and the like.

FIG. 29B is an external perspective view showing the bottom surface sideof the package. On the bottom surface of the package, a ball grid array(BGA) including solder balls as bumps 840 is formed. Although the BGA isemployed here, a land grid array (LGA), a pin grid array (PGA), or thelike may be alternatively employed.

FIG. 29C is a perspective view of the package, in which the cover glass820 and the adhesive 830 are partly illustrated. FIG. 29D is across-sectional view of the package. Electrode pads 860 are formed overthe package substrate 810, and electrically connected to the bumps 840through through-holes 880 and lands 885. The electrode pads 860 areelectrically connected to electrodes of the image sensor chip 850through wires 870.

FIG. 30A is an external perspective view showing the top surface side ofa camera module in which an image sensor chip is mounted on a packagewith a built-in lens. The camera module includes a package substrate 811to which an image sensor chip 851 is fixed, a lens cover 821, a lens835, and the like. Furthermore, an IC chip 890 having functions of adriver circuit, a signal conversion circuit, and the like of an imagingdevice is provided between the package substrate 811 and the imagesensor chip 851. Thus, a system in package (SiP) is formed.

FIG. 30B is an external perspective view showing the bottom surface sideof the camera module. On the bottom surface and four side surfaces ofthe package substrate 811, mounting lands 841 are provided; thisstructure can be called a quad flat no-lead package (QFN). Although QFNis employed here, a quad flat package (QFP), the above BGA, or the likemay be alternatively employed.

FIG. 30C is a perspective view of the module, in which the lens cover821 and the lens 835 are partly illustrated. FIG. 30D is across-sectional view of the camera module. The lands 841 are partly usedas electrode pads 861. The electrode pads 861 are electrically connectedto electrodes of the image sensor chip 851 and the IC chip 890 throughwires 871.

The image sensor chip placed in the package having the above structurecan be easily mounted on and incorporated into a variety ofsemiconductor devices and electronic devices.

The structures described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 8

Examples of an electronic device that can use the imaging deviceaccording to one embodiment of the present invention include displaydevices, personal computers, image memory devices or image reproducingdevices provided with storage media, mobile phones, game machines(including portable game machines), portable data terminals, e-bookreaders, cameras such as video cameras and digital still cameras,goggle-type displays (head mounted displays), navigation systems, audioreproducing devices (e.g., car audio players and digital audio players),copiers, facsimiles, printers, multifunction printers, automated tellermachines (ATM), and vending machines. FIGS. 31A to 31F illustratespecific examples of these electronic devices.

FIG. 31A illustrates a monitoring camera, which includes a housing 951,a lens 952, a support portion 953, and the like. The imaging device ofone embodiment of the present invention can be included as a componentfor obtaining an image in the monitoring camera. Note that a “monitoringcamera” is a common name and does not limit the uses. For example, adevice that has a function of a monitoring camera can also be called acamera or a video camera.

FIG. 31B illustrates a video camera, which includes a first housing 971,a second housing 972, a display portion 973, operation keys 974, a lens975, a joint 976, and the like. The operation keys 974 and the lens 975are provided for the first housing 971, and the display portion 973 isprovided for the second housing 972. The imaging device of oneembodiment of the present invention can be included as a component forobtaining an image in the video camera.

FIG. 31C illustrates a digital camera, which includes a housing 961, ashutter button 962, a microphone 963, a light-emitting portion 967, alens 965, and the like. The imaging device of one embodiment of thepresent invention can be included as a component for obtaining an imagein the digital camera.

FIG. 31D illustrates a wrist-watch-type information terminal, whichincludes a housing 931, a display portion 932, a wristband 933,operation buttons 935, a winder 936, a camera 939, and the like. Thedisplay portion 932 may be a touch panel. The imaging device of oneembodiment of the present invention can be included as a component forobtaining an image in the information terminal.

FIG. 31E illustrates a portable game machine, which includes housings901 and 902, display portions 903 and 904, a microphone 905, speakers906, an operation key 907, a stylus 908, a camera 909, and the like.Although the portable game machine in FIG. 31E has the two displayportions 903 and 904, the number of display portions included in aportable game machine is not limited to this. The imaging device of oneembodiment of the present invention can be included as one component forobtaining an image in the portable game machine.

FIG. 31F illustrates a portable data terminal, which includes a housing911, a display portion 912, a camera 919, and the like. A touch panelfunction of the display portion 912 enables input and output ofinformation. The imaging device of one embodiment of the presentinvention can be included as one component for obtaining an image in theportable data terminal.

REFERENCE NUMERALS

a1: output signal, a2: output signal, b1: output signal, c1: outputsignal, C1: capacitor, C2: capacitor, C3: capacitor, CN1: countercircuit, CN2: counter circuit, cnt1: output signal, Fn1: floating node,G1: scan line, G2: scan line, OUT1: signal line, R1: resistor, Vbias2:terminal, Wd1: signal line, Wd2: signal line, 10: imaging element, 12:circuit, 20: pixel, 20 a: determination circuit, 20A: pixel, 20B: pixel,20C: pixel, 21: light-receiving circuit, 21 a: light-receiving circuit,22: amplifier circuit, 22 a: amplifier circuit, 22 b: memory circuit, 22c: adder circuit, 23: memory circuit, 26: A/D converter circuit, 26 a:comparator, 26 b: counter circuit, 27: decoder circuit, 28: selectorcircuit, 29: control portion, 30: feature extraction circuit, 30 a:operational amplifier, 30N: synapse circuit, 31: determination outputcircuit, 31 a: arithmetic circuit, 31 b: memory circuit, 31N: activationfunction circuit, 32: feature extraction circuit, 32 a: input selectioncircuit, 32 b: inverter, 32 c: counter circuit, 32N: synapse circuit,33: output circuit, 33 a: determination circuit, 33 b: circuit, 33 c:memory circuit, 37: terminal, 38: terminal, 41: transistor, 42:transistor, 43: transistor, 44: transistor, 44 a: transistor, 44 b:transistor, 45 a: transistor, 45 b: transistor, 46: transistor, 47:transistor, 48: transistor, 49: transistor, 69: wiring, 80: insulatinglayer, 81: insulating layer, 81 a: insulating layer, 81 b: insulatinglayer, 81 e: insulating layer, 81 g: insulating layer, 81 h: insulatinglayer, 82: conductor, 82 a: conductor, 82 b: conductor, 93: wiring, 100:imaging device, 101: transistor, 102: transistor, 103: transistor, 104:transistor, 105: transistor, 106: transistor, 107: transistor, 115:substrate, 120: insulating layer, 130: oxide semiconductor layer, 130 a:oxide semiconductor layer, 130 b: oxide semiconductor layer, 130 c:oxide semiconductor layer, 130 d: oxide semiconductor layer, 140:conductive layer, 145: insulating layer, 150: conductive layer, 155:insulating layer, 160: insulating layer, 170: conductive layer, 171:conductive layer, 172: conductive layer, 173: conductive layer, 180:insulating layer, 200: conductor, 201: conductor, 210: insulating layer,231: region, 232: region, 300: amplifier circuit, 301: input selectioncircuit, 302: A/D converter circuit, 303: determination circuit, 304:memory circuit, 305: selection circuit, 306: logic circuit, 310:determination output circuit, 405: metal layer, 406: metal layer, 561:photoelectric conversion layer, 562: light-transmitting conductivelayer, 563: semiconductor layer, 564: semiconductor layer, 565:semiconductor layer, 566: electrode, 566 a: conductive layer, 566 b:conductive layer, 567: partition wall, 568: hole-injection blockinglayer, 569: electron-injection blocking layer, 571: wiring, 571 a:conductive layer, 571 b: conductive layer, 588: wiring, 600: siliconsubstrate, 620: p⁺ region, 630: p⁻ region, 640: n-type region, 650: p⁺region, 660: semiconductor layer, 810: package substrate, 811: packagesubstrate, 820: cover glass, 821: lens cover, 830: adhesive, 835: lens,840: bump, 841: land, 850: image sensor chip, 851: image sensor chip,860: electrode pad, 861: electrode pad, 870: wire, 871: wire, 880:through-hole, 885: land, 890: IC chip, 901: housing, 902: housing, 903:display portion, 904: display portion, 905: microphone, 906: speaker,907: operation key, 908: stylus, 909: camera, 911: housing, 912: displayportion, 919: camera, 931: housing, 932: display portion, 933:wristband, 935: button, 936: winder, 939: camera, 951: housing, 952:lens, 953: support portion, 961: housing, 962: shutter button, 963:microphone, 965: lens, 967: light-emitting portion, 971: housing, 972:housing, 973: display portion, 974: operation key, 975: lens, 976:joint, 1530: light-blocking layer, 1540: microlens array, 1550 a:optical conversion layer, 1550 b: optical conversion layer, and 1550 c:optical conversion layer.

This application is based on Japanese Patent Application Serial No.2016-153192 filed with Japan Patent Office on Aug. 3, 2016, and JapanesePatent Application serial No. 2016-153194 filed with Japan Patent Officeon Aug. 3, 2016, the entire contents of which are hereby incorporated byreference.

1. An imaging device comprising a plurality of circuits, wherein theplurality of circuits each comprises: a plurality of pixels; and a firstcircuit configured to perform an arithmetic operation using a pluralityof first signals output from the plurality of pixels, wherein theplurality of pixels each comprises: a second circuit configured togenerate a second signal; a third circuit configured to retain apotential corresponding to a weight coefficient; and an operationcircuit configured to receive the potential corresponding to the weightcoefficient and to multiply the second signal by the weight coefficient,so that one of the first signals is generated, wherein the secondcircuit comprises a first transistor, wherein the third circuitcomprises a second transistor and a capacitor, wherein the operationcircuit comprises a third transistor, and wherein the operation circuitis electrically connected to one of a source and a drain of the secondtransistor and one electrode of the capacitor.
 2. The imaging deviceaccording to claim 1, wherein the second circuit further comprises aphotoelectric conversion element, wherein the first transistor isconfigured to control an output of the second signal in accordance withan output single from the photoelectric conversion element, and whereina layer comprising the first transistor is located over a layercomprising the first circuit, and the photoelectric conversion elementis located over the layer comprising the first transistor.
 3. An imagingdevice comprising a plurality of pixels; wherein the plurality of pixelseach comprises: a first circuit configured to generate a first signal; asecond circuit configured to retain a potential corresponding to aweight coefficient; and an operation circuit configured to receive thepotential corresponding to the weight coefficient and to multiply thefirst signal by the weight coefficient, so that an output signal isgenerated, wherein the first circuit comprises a first transistor,wherein the second circuit comprises a second transistor and acapacitor, wherein the operation circuit comprises a third transistor,and wherein the operation circuit is electrically connected to one of asource and a drain of the second transistor and one electrode of thecapacitor.
 4. The imaging device according to claim 3, wherein the firstcircuit further comprises a photoelectric conversion element, whereinthe first transistor is configured to control an output of the firstsignal in accordance with an output single from the photoelectricconversion element, and wherein a layer comprising the first transistoris located over a layer comprising the operation circuit, and thephotoelectric conversion element is located over the layer comprisingthe first transistor.
 5. The imaging device according to claim 1,wherein the operation circuit comprises at least a part of a neuralnetwork.
 6. The imaging device according to claim 3, wherein theoperation circuit comprises at least a part of a neural network.